Fair Value Measurements (Details 2) (USD $)			3 Months Ended		9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Derivative instruments, at fair value, net	Sep. 30, 2012 Derivative instruments, at fair value, net	Sep. 30, 2013 Derivative instruments, at fair value, net	Sep. 30, 2012 Derivative instruments, at fair value, net
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation
Balance at the beginning of the period			$ 74,688,000	$ 129,436,000	$ 90,714,000	$ 143,260,000
Total gains or losses included in earnings			414,000	(90,000)	(341,000)	(90,000)
Total gains or losses included in other comprehensive income			2,067,000	(13,839,000)	3,459,000	13,554,000
Purchases			(79,000)	22,000	(7,000)	(994,000)
Settlements			(8,769,000)	(18,558,000)	(25,504,000)	(58,759,000)
Balance at the end of the period			68,321,000	96,971,000	68,321,000	96,971,000
Unrealized gains and losses included in earnings			400,000	0	(800,000)	0
Estimated fair value of long-term debt	$ 2,800,000,000	$ 2,900,000,000