Fair Value Measurements (Details 2) (USD $)			3 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Derivative instruments, at fair value, net	Jun. 30, 2012 Derivative instruments, at fair value, net	Jun. 30, 2013 Derivative instruments, at fair value, net	Jun. 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			$ 69,865,000	$ 156,066,000	$ 90,714,000	$ 143,260,000
Total gains or losses included in earnings			(1,178,000)		(755,000)
Total gains or losses included in other comprehensive income			13,125,000	(4,203,000)	1,392,000	27,393,000
Purchases				(1,016,000)	72,000	(1,016,000)
Settlements			(7,124,000)	(21,411,000)	(16,735,000)	(40,201,000)
Balance at the end of the period			74,688,000	129,436,000	74,688,000	129,436,000
Unrealized losses included in earnings			1,200,000		1,200,000
Estimated fair value of long-term debt	$ 2,800,000,000	$ 2,900,000,000