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OTHER INFORMATION (Details 5) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Details of standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves      
Future cash inflows $ 4,248,384 $ 2,409,249 $ 709,788
Future production costs (1,520,260) (765,048) (253,544)
Future development costs (603,809) (330,007) (77,216)
Future income tax expense (230,500) (253,721) (88,233)
Future net cash flows 1,893,815 1,060,473 290,795
10% annual discount for estimated timing of cash flows (1,046,162) (586,077) (162,836)
Standardized measure of discounted future net cash flows $ 847,653 $ 474,396 $ 127,959