Fair Value Measurements (Details 2) (USD $)			3 Months Ended		9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Derivative instruments, at fair value, net	Sep. 30, 2011 Derivative instruments, at fair value, net	Sep. 30, 2012 Derivative instruments, at fair value, net	Sep. 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			$ 129,436,000	$ 101,972,000	$ 143,260,000	$ 116,672,000
Total gains or losses included in earnings			(90,000)		(90,000)	14,000
Total gains or losses included in other comprehensive income			(13,839,000)	24,101,000	13,554,000	35,121,000
Purchases			22,000		(994,000)
Settlements			(18,558,000)	(12,989,000)	(58,759,000)	(38,723,000)
Balance at the end of the year			96,971,000	113,084,000	96,971,000	113,084,000
Estimated fair value of long-term debt	$ 3,100,000,000	$ 3,000,000,000