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, 2012
Dec. 31, 2011
Dec. 31, 2010
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows [Abstract]      
Future cash inflows $ 1,115,570 $ 1,194,079 $ 804,111
Production (249,329) (356,653) (277,793)
Development and net abandonment (273,817) (268,628) (146,870)
Future net inflows before income taxes 592,424 568,798 379,448
Future income taxes (55,772) (78,813) (24,719)
Future net cash flows 536,652 489,985 354,729
10% discount factor (305,504) (219,628) (155,813)
Standardized measure of discounted future net cash flows 231,148 270,357 198,916
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 270,357 198,916 135,921
Sales and transfers, net of production costs (84,044) (107,297) (72,171)
Net change in sales and transfer prices, net of production costs 47,261 125,518 126,571
Net change due to purchases and sales of in place reserves (35,665) 1,275 621
Extensions, discoveries, and improved recovery, net of future production and development costs incurred 53,446 22,598 23,739
Changes in future development cost 39,815 (83,110) (68,960)
Revisions of quantity estimates (77,322) (949) 23,295
Accretion of discount 30,989 68,384 10,597
Net change in income taxes 13,969 (32,918) (5,170)
Changes in production rates, timing and other (27,658) 77,940 24,473
Aggregate change (39,209) 71,441 62,995
Standardized measure at the end of the period $ 231,148 $ 270,357 $ 198,916