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Fair Value Measurements (Details 2) (USD $)
3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2012
Derivative instruments, at fair value, net
Mar. 31, 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     $ 143,260,000 $ 116,672,000
Total gains or losses included in earnings       14,000
Total gains or losses included in other comprehensive income     31,596,000 (5,831,000)
Settlements     (18,790,000) (13,491,000)
Balance at the end of the year     156,066,000 97,364,000
Estimated fair value of long-term debt $ 3,000,000,000 $ 3,000,000,000