Derivatives and Risk Mangement (Balance Sheet Classification) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Total derivative assets	$ 240,856	$ 208,325
Total derivative liabilities	37,696	47,873
Designated as Hedging Instrument [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	240,522	208,292
Derivative Liability Designated as Hedging Instrument, Fair Value	37,196	45,865
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	123,608	81,797
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Other Assets
Derivative Asset Designated as Hedging Instrument, Fair Value	19,717	5,086
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	2,266	1,774
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Long-Term Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	27,294	22,334
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	59,532	48,582
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Other Assets
Derivative Asset Designated as Hedging Instrument, Fair Value	37,665	72,827
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	2,374	3,903
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Long-Term Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	5,262	17,854
Not Designated as Hedging Instrument [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	334	33
Derivative Liability Not Designated as Hedging Instrument, Fair Value	500	2,008
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Asset [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	138	33
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Assets
Derivative Liability Not Designated as Hedging Instrument, Fair Value	196	0
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Liability [Member]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	240	2,008
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Long-Term Hedging Liability [Member]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	$ 260	$ 0