Annual report pursuant to Section 13 and 15(d)

Supplemental Oil and Natural Gas Reserve Data (unaudited) (Proved Developed and Undeveloped Reserves) (Details)

v2.4.1.9
Supplemental Oil and Natural Gas Reserve Data (unaudited) (Proved Developed and Undeveloped Reserves) (Details)
12 Months Ended
Dec. 31, 2014
MBbls
Dec. 31, 2013
MBbls
Dec. 31, 2012
MBbls
Dec. 31, 2011
MBoe
Proved developed and undeveloped reserves:        
Beginning of period, MBOE proved developed       7,003us-gaap_ProvedDevelopedReservesBOE1
End of period, MBOE proved developed 18,201us-gaap_ProvedDevelopedReservesBOE1 7,470us-gaap_ProvedDevelopedReservesBOE1 6,735us-gaap_ProvedDevelopedReservesBOE1 7,003us-gaap_ProvedDevelopedReservesBOE1
Beginning of periodc, MBOE proved developed       8,925us-gaap_ProvedUndevelopedReserveBOE1
End of period, MBOE proved developed 14,623us-gaap_ProvedUndevelopedReserveBOE1 7,387us-gaap_ProvedUndevelopedReserveBOE1 7,337us-gaap_ProvedUndevelopedReserveBOE1 8,925us-gaap_ProvedUndevelopedReserveBOE1
Crude Oil [Member]        
Proved developed and undeveloped reserves:        
Beginning of period 11,898us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
10,780us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
10,075us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
Revisions to previous estimates (243)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
(2,540)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
(488)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
Purchase of reserves in place 3,223us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
150us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
38us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
Sale of reserves in place   (3,294)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
(504)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
Extensions and discoveries 12,547us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
7,713us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
2,636us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
Production (1,692)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
(911)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
(977)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
End of period 25,733us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
11,898us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
10,780us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
Beginning of period, proved developed 5,960us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
4,955us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
5,069us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
End of period, proved developed 14,006us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
5,960us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
4,955us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
Beginning of period, proved undeveloped 5,938us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
5,825us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
5,006us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
End of period, proved undeveloped 11,727us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
5,938us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
5,825us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_CrudeOilMember
 
Natural Gas [Member]        
Proved developed and undeveloped reserves:        
Beginning of period 17,751us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
19,753us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
35,118us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
Revisions to previous estimates (215)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
(5,351)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
(10,838)us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
Purchase of reserves in place 8,591us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
317us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
115us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
Sale of reserves in place   (4,576)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
(4,404)us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
Extensions and discoveries 18,641us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
10,619us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
3,350us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
Production (2,220)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
(3,011)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
(3,588)us-gaap_ProvedDevelopedAndUndevelopedReservesProduction
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
End of period 42,548us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
17,751us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
19,753us-gaap_ProvedDevelopedAndUndevelopedReservesNet
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
Beginning of period, proved developed 9,059us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
10,680us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
11,605us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
End of period, proved developed 25,171us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
9,059us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
10,680us-gaap_ProvedDevelopedReservesVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
Beginning of period, proved undeveloped 8,692us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
9,073us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
23,513us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
 
End of period, proved undeveloped 17,377us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
8,692us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember
9,073us-gaap_ProvedUndevelopedReserveVolume
/ us-gaap_ReserveQuantitiesByTypeOfReserveAxis
= us-gaap_NaturalGasReservesMember