Oil and Gas Operations (Unaudited) (Standardized Measure of Discounted Future Net Cash Flows) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Deferred hedgin gain (loss) excluded from calculation of standardized measure of future net cash flows	$ 15,000,000	$ (70,400,000)	$ 79,700,000
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Standardized Measure [Abstract]
Future gross revenues	18,196,229,000	13,210,211,000	8,208,613,000
Future production costs	5,823,395,000	4,959,403,000	3,915,736,000
Future development costs	1,539,072,000	1,026,903,000	533,674,000
Future income tax expense	3,326,382,000	2,201,742,000	944,875,000
Future net cash flows	7,507,380,000	5,022,163,000	2,814,328,000
Discount at 10% per annum	3,878,217,000	2,555,027,000	1,251,138,000
Standardized measure of discounted future net cash flows relating to proved oil and gas reserves	3,629,163,000	2,467,136,000	1,563,190,000
Discounted future net cash flows before income taxes	4,691,086,000	3,155,746,000	1,765,632,000
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	2,467,136,000	1,563,190,000	1,626,617,000
Revisions to reserves proved in prior years:
Net changes in prices, production costs and future development costs	707,411,000	945,179,000	(248,236,000)
Net changes due to revisions in quantity estimates	(80,004,000)	36,349,000	(117,990,000)
Development costs incurred, previously estimated	392,720,000	195,269,000	140,169,000
Accretion of discount	246,714,000	156,319,000	162,662,000
Changes in timing and other	(25,937,000)	15,815,000	97,142,000
Total revisions	1,240,904,000	1,348,931,000	33,747,000
New field discoveries and extensions, net of future production and development costs	755,977,000	319,223,000	81,954,000
Sales of oil and gas produced, net of production costs	(763,171,000)	(576,755,000)	(389,125,000)
Purchases	232,768,000	278,384,000	116,435,000
Sales	0	87,000	(7,571,000)
Net change in income taxes	(304,451,000)	(465,924,000)	101,133,000
Net change in standardized measure of discounted future net cash flows	1,162,027,000	903,946,000	(63,427,000)
Balance at end of year	$ 3,629,163,000	$ 2,467,136,000	$ 1,563,190,000