Annual report pursuant to Section 13 and 15(d)

Supplemental Oil and Natural Gas Reserve Data (unaudited) (Capitalized Costs) (Details)

v2.4.0.8
Supplemental Oil and Natural Gas Reserve Data (unaudited) (Capitalized Costs) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Evaluated Properties-      
Beginning of period balance $ 1,497,010 $ 1,421,640 $ 1,316,677
Capitalized G&A 10,014 12,148 11,205
Property acquisition costs 10,885 2,075 0
Exploration costs 147,164 22,703 5,473
Development costs 36,504 38,444 88,285
End of period balance 1,701,577 1,497,010 1,421,640
Unevaluated Properties (excluded from amortization):      
Beginning of period balance 68,776 2,603 8,106
Acquisitions 2,259 29,590 2,422
Exploration 10,767 34,674 1,372
Capitalized interest 4,410 2,109 573
Transfers to evaluated (42,990) (200) (9,870)
End of period balance 43,222 68,776 2,603
Accumulated depreciation, depletion and amortization:      
Beginning of period balance 1,296,265 1,208,331 1,155,915
Provision charged to expense 42,251 48,524 52,416
Sale of mineral interests 82,096 39,410 0
End of period balance 1,420,612 1,296,265 1,208,331
Capitalized Costs of Unproved Properties Excluded from Amortization, Increase (Decrease) to Period Cost $ (25,554)    
Depletion Expense Per Physical Unit of Production 31.12 31.56 26.42
Lease Expense Per Physical Unit of Production 14.00 14.81 9.92
Minimum [Member]
      
Accumulated depreciation, depletion and amortization:      
Unevaluated Property Costs Expected Evaluation Period Range 3 years    
Maximum [Member]
      
Accumulated depreciation, depletion and amortization:      
Unevaluated Property Costs Expected Evaluation Period Range 5 years