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Supplemental Information on Oil and Natural Gas Producing Activities - Estimate of Changes in Standardized Measure of Discounted Future Net Cash Flows from Proved Reserves (Unaudited) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Standardized Measure [Abstract]      
Present value, beginning balance $ 5,840,368 [1] $ 5,216,337 [1] $ 3,683,515
Changes during the year      
Revenues less production and other costs (1,271,559) (1,234,918) (857,848)
Net changes in prices, production and other costs 271,566 (2,555,391) 1,264,736
Development costs incurred 474,275 766,943 575,546
Net changes in future development costs (207,729) (45,397) 87,080
Extensions and discoveries 1,406,102 2,092,423 1,812,167
Revisions of previous quantity estimates (296,418) (530,755) (345,965)
Accretion of discount 711,385 678,200 455,501
Net change in income taxes 477,328 11,433 (833,841)
Purchases of reserves in-place 1,628 1,708,301 44,934
Sales of reserves in-place (3,172,187) (410,415) (558,257)
Timing differences and other (217,148) [2] 143,607 [2] (111,231) [2]
Net change for the year (1,822,757) 624,031 1,532,822
Present value, ending balance $ 4,017,611 [1] $ 5,840,368 [1] $ 5,216,337 [1]
[1] Includes approximately $781.6 million, $952.7 million and $932.8 million attributable to noncontrolling interests at December 31, 2013, 2012, and 2011 respectively.
[2] The change in timing differences and other are related to revisions in the Company’s estimated time of production and development.