Annual report pursuant to Section 13 and 15(d)

Supplemental Oil and Natural Gas Reserve Data (unaudited) (Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves) (Details)

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Supplemental Oil and Natural Gas Reserve Data (unaudited) (Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2009
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows [Abstract]        
Future cash inflows $ 1,193,299 $ 1,115,570 $ 1,194,079  
Production (357,005) (249,329) (356,653)  
Development and net abandonment (155,667) (273,817) (268,628)  
Future net inflows before income taxes 680,627 592,424 568,798  
Future income taxes (68,239) (55,772) (78,813)  
Future net cash flows 612,388 536,652 489,985  
10% discount factor (328,442) (305,504) (219,628)  
Standardized measure of discounted future net cash flows 283,946 231,148 270,357  
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]        
Standardized measure at the beginning of the period 231,148 270,357   198,916
Sales and transfers, net of production costs (78,661) (84,044) (107,297)  
Net change in sales and transfer prices, net of production costs (46,088) 47,261 125,518  
Net change due to purchases and sales of in place reserves (145,711) (35,665) 1,275  
Extensions, discoveries, and improved recovery, net of future production and development costs incurred 212,431 53,446 22,598  
Changes in future development cost 153,983 39,815 (83,110)  
Revisions of quantity estimates (68,958) (77,322) (949)  
Accretion of discount 25,010 30,989 68,384  
Net change in income taxes 1,751 13,969 (32,918)  
Changes in production rates, timing and other (959) (27,658) 77,940  
Aggregate change 52,798 (39,209) 71,441  
Standardized measure at the end of the period $ 283,946 $ 231,148 $ 270,357 $ 198,916