Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Restricted cash	$ 69,261		$ 69,261		$ 0
Derivative instruments, net	398,916		398,916		160,452
Derivative assets	412,569		412,569		208,325
Derivative liabilities	(13,653)		(13,653)		(47,873)
Total derivative instruments, net	398,916		398,916		160,452
Balance at beginning of period	89,395	53,566	97,677	24,720
Total gains or losses (realized/unrealized): Included in earnings	11,102	14,956	38,694	48,743
Total gains or losses (realized/unrealized): Included in other comprehensive income (loss)	35,223	68,834	23,913	91,224
Purchases, issuances, and settlements:
Purchases	0	0	0	0
Issuances	0	0	0	0
Settlements	(13,895)	(13,493)	(38,601)	(40,824)
Transfers into/out of Level 3	0	0	142	0
Balance at end of period	121,825	123,863	121,825	123,863
Change in unrealized gain (loss) included in earnings relating to derivatives still held as of September 30	(2,793)	1,463	93	7,919
Carrying Amount [Member]
Cash and cash equivalents	13,293		13,293		16,055
Restricted cash	69,261		69,261		0
Unsecured revolving credit facility	599,800		599,800		421,200
Senior notes	672,400		672,400		673,000
Derivative instruments, net	398,916		398,916		160,452
Total derivative instruments, net	398,916		398,916		160,452
Fair Value [Member]
Cash and cash equivalents	13,293		13,293		16,055
Restricted cash	69,261		69,261		0
Unsecured revolving credit facility	599,800		599,800		421,200
Senior notes	774,915		774,915		761,372
Derivative instruments, net	398,916		398,916		160,452
Total derivative instruments, net	398,916		398,916		160,452
Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Derivative instruments, net	0		0		0
Derivative assets	0		0		0
Derivative liabilities	0		0		0
Total derivative instruments, net	0		0		0
Significant Observable Inputs, Level 2 [Member]
Derivative instruments, net	277,091		277,091		62,775
Derivative assets	286,131		286,131		86,883
Derivative liabilities	(9,040)		(9,040)		(24,108)
Total derivative instruments, net	277,091		277,091		62,775
Significant Unobservable Inputs, Level 3 [Member]
Derivative instruments, net	121,825		121,825		97,677
Derivative assets	126,438		126,438		121,442
Derivative liabilities	(4,613)		(4,613)		(23,765)
Total derivative instruments, net	121,825		121,825		97,677
7.35% Senior Notes due 2017 [Member]
Debt instrument, interest rate, stated percentage	7.35%		7.35%
7.5% Senior Notes due 2018 [Member]
Debt instrument, interest rate, stated percentage	7.50%		7.50%
Debt instrument maturity			2018
Yield, Publicly-traded debt market	4.70%		4.70%		5.20%
7.125% Senior Notes due 2017 [Member]
Debt instrument, interest rate, stated percentage	7.125%		7.125%
7.15% Senior Notes due 2018 [Member]
Debt instrument, interest rate, stated percentage	7.15%		7.15%
Parent [Member]
Restricted cash	$ 69,261		$ 69,261

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