Fair Value Measurements (Details 2) (USD $)			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Derivative instruments, at fair value, net	Mar. 31, 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			$ 90,714,000	$ 143,260,000
Total gains or losses included in earnings			423,000
Total gains or losses included in other comprehensive income			(11,733,000)	31,596,000
Purchases			(72,000)
Settlements			(9,611,000)	(18,790,000)
Balance at the end of the period			69,865,000	156,066,000
Unrealized gains or losses included in earnings			500,000
Estimated fair value of long-term debt	$ 2,900,000,000	$ 2,900,000,000