Fair Value Measurements (Summary Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Derivative assets
Derivative liabilities
Total
Significant Observable Inputs, Level 2 [Member]
Derivative assets	465,571	534,560
Derivative liabilities	(7,086)	(11,849)
Total	458,485	522,711
Significant Unobservable Inputs, Level 3 [Member]
Derivative assets	106,391	182,783
Derivative liabilities	(169)	(664)
Total	106,222	182,119
Fair Value [Member]
Derivative assets	571,962	717,343
Derivative liabilities	(7,255)	(12,513)
Total	$ 564,707	$ 704,830