Fair Value Measurements (Details 2) (USD $)			3 Months Ended		6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Derivative instruments, at fair value, net	Jun. 30, 2011 Derivative instruments, at fair value, net	Jun. 30, 2012 Derivative instruments, at fair value, net	Jun. 30, 2011 Derivative instruments, at fair value, net
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation
Balance at the beginning of the year			$ 156,066,000	$ 97,364,000	$ 143,260,000	$ 116,672,000
Total gains or losses included in earnings						14,000
Total gains or losses included in other comprehensive income			(4,203,000)	16,851,000	27,393,000	11,020,000
Purchases			(1,016,000)		(1,016,000)
Settlements			(21,411,000)	(12,243,000)	(40,201,000)	(25,734,000)
Balance at the end of the year			129,436,000	101,972,000	129,436,000	101,972,000
Estimated fair value of long-term debt	$ 3,100,000,000	$ 3,100,000,000