Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Balance at beginning of period	$ 97,677	$ 24,720
Total gains or losses (realized/unrealized): Included in earnings	67,998	68,111
Total gains or losses (realized/unrealized): Included in other comprehensive income (loss)	79,215	63,522
Purchases, issuances, and settlements:
Settlements	(62,913)	(58,676)
Transfers into/out of Level 3	142
Balance at end of period	182,119	97,677
Change in unrealized gain (loss) included in earnings relating to derivatives still held as of December 31	5,085	9,435
7.5% Senior Notes due 2018 [Member]
Debt instrument, interest rate, stated percentage	7.50%
Debt instrument maturity	2018
Yield, Publicly-traded debt market	4.60%	5.20%
Carrying Amount [Member]
Cash and cash equivalents	15,627	16,055
Unsecured revolving credit facility	671,500	421,200
Senior notes	671,800	673,000
Total derivative instruments, net	704,830	160,452
Fair Value [Member]
Cash and cash equivalents	15,627	16,055
Unsecured revolving credit facility	671,500	421,200
Senior notes	773,578	761,372
Derivative assets	717,343	208,325
Derivative liabilities	(12,513)	(47,873)
Total derivative instruments, net	704,830	160,452
Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Derivative assets	0	0
Derivative liabilities	0	0
Total derivative instruments, net	0	0
Significant Observable Inputs, Level 2 [Member]
Derivative assets	534,560	86,883
Derivative liabilities	(11,849)	(24,108)
Total derivative instruments, net	522,711	62,775
Significant Unobservable Inputs, Level 3 [Member]
Derivative assets	182,783	121,442
Derivative liabilities	(664)	(23,765)
Total derivative instruments, net	$ 182,119	$ 97,677