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Supplemental Oil and Gas Reserve Information - Standardized Measure of Discounted Future Cash Flows (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Oil and Gas Exploration and Production Industries Disclosures [Abstract]      
Future cash inflows $ 19,063,500 $ 11,321,992 $ 6,508,604
Future production costs (5,473,767) (2,809,960) (1,690,264)
Future development costs (1,904,095) (1,434,648) (783,486)
Future income tax expense (3,628,977) (2,123,973) (1,225,395)
Future net cash flows 8,056,661 4,953,411 2,809,459
10% annual discount for estimated timing of cash flows (4,329,102) (2,693,514) (1,489,988)
Standardized measure of discounted future net cash flows $ 3,727,559 $ 2,259,897 $ 1,319,471