Oil and Gas Operations (Unaudited) (Standardized Measure of Discounted Future Net Cash Flows) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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	$ 74,800,000	$ 15,000,000	$ 70,400,000
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Standardized Measure [Abstract]
Future gross revenues	17,735,363,000	18,196,229,000	13,210,211,000
Future production costs	5,715,248,000	5,823,395,000	4,959,403,000
Future development costs	1,892,600,000	1,539,072,000	1,026,903,000
Future income tax expense	2,809,411,000	3,326,382,000	2,201,742,000
Future net cash flows	7,318,104,000	7,507,380,000	5,022,163,000
Discount at 10% per annum	3,618,785,000	3,878,217,000	2,555,027,000
Standardized measure of discounted future net cash flows relating to proved oil and gas reserves	3,699,319,000	3,629,163,000	2,467,136,000
Discounted future net cash flows before income taxes	4,411,399,000	4,691,086,000	3,155,746,000
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	3,629,163,000	2,467,136,000	1,563,190,000
Revisions to reserves proved in prior years:
Net changes in prices, production costs and future development costs	(922,792,000)	707,411,000	945,179,000
Net changes due to revisions in quantity estimates	(383,755,000)	(80,004,000)	36,349,000
Development costs incurred, previously estimated	472,603,000	392,720,000	195,269,000
Accretion of discount	362,916,000	246,714,000	156,319,000
Changes in timing and other	(317,244,000)	(25,937,000)	15,815,000
Total revisions	(788,272,000)	1,240,904,000	1,348,931,000
New field discoveries and extensions, net of future production and development costs	1,025,419,000	755,977,000	319,223,000
Sales of oil and gas produced, net of production costs	(812,781,000)	(763,171,000)	(576,755,000)
Purchases	189,755,000	232,768,000	278,384,000
Sales	0	0	87,000
Net change in income taxes	456,035,000	(304,451,000)	(465,924,000)
Net change in standardized measure of discounted future net cash flows	70,156,000	1,162,027,000	903,946,000
Balance at end of year	$ 3,699,319,000	$ 3,629,163,000	$ 2,467,136,000