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Oil and Gas Operations (Unaudited) (Standardized Measure of Discounted Future Net Cash Flows) (Details) (USD $)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Oil and Gas Exploration and Production Industries Disclosures [Abstract]      
Deferred hedging gain (loss) excluded from calculation of standardized measure of future net cash flows $ 15,000,000 $ 21,600,000 $ 74,800,000
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Standardized Measure [Abstract]      
Future gross revenues 19,509,305,000 17,735,363,000 18,196,229,000
Future production costs 6,136,709,000 5,715,248,000 5,823,395,000
Future development costs 1,896,602,000 1,892,600,000 1,539,072,000
Future income tax expense 3,209,697,000 2,809,411,000 3,326,382,000
Future net cash flows 8,266,297,000 7,318,104,000 7,507,380,000
Discount at 10% per annum 4,248,456,000 3,618,785,000 3,878,217,000
Standardized measure of discounted future net cash flows relating to proved oil and gas reserves 4,017,841,000 3,699,319,000 3,629,163,000
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]      
Balance at beginning of year 3,699,319,000 3,629,163,000 2,467,136,000
Revisions to reserves proved in prior years:      
Net changes in prices, production costs and future development costs 566,838,000 (922,792,000) 707,411,000
Net changes due to revisions in quantity estimates (81,762,000) (383,755,000) (80,004,000)
Development costs incurred, previously estimated 299,432,000 472,603,000 392,720,000
Accretion of discount 369,932,000 362,916,000 246,714,000
Changes in timing and other (179,502,000) (317,244,000) (25,937,000)
Total revisions 974,938,000 (788,272,000) 1,240,904,000
New field discoveries and extensions, net of future production and development costs 376,326,000 1,025,419,000 755,977,000
Sales of oil and gas produced, net of production costs (1,014,593,000) (812,781,000) (763,171,000)
Purchases 4,690,000 189,755,000 232,768,000
Sales (24,876,000) 0 0
Net change in income taxes 2,037,000 456,035,000 (304,451,000)
Net change in standardized measure of discounted future net cash flows 318,522,000 70,156,000 1,162,027,000
Balance at end of year $ 4,017,841,000 $ 3,699,319,000 $ 3,629,163,000