Derivatives and Risk Mangement (Balance Sheet Classification) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Total derivative assets	$ 412,569	$ 208,325
Total derivative liabilities	13,653	47,873
Designated as Hedging Instrument [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	411,787	208,292
Derivative Liability Designated as Hedging Instrument, Fair Value	10,738	45,865
Designated as Hedging Instrument [Member] \| Fixed and Floating Price Swaps [Member] \| Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	213,334	81,797
Designated as Hedging Instrument [Member] \| Fixed and Floating Price Swaps [Member] \| Other Assets
Derivative Asset Designated as Hedging Instrument, Fair Value	72,797	5,086
Designated as Hedging Instrument [Member] \| Fixed and Floating Price Swaps [Member] \| Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	6,533	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	2,507	22,334
Designated as Hedging Instrument [Member] \| Costless Collar [Member] \| Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	101,988	48,582
Designated as Hedging Instrument [Member] \| Costless Collar [Member] \| Other Assets
Derivative Asset Designated as Hedging Instrument, Fair Value	23,668	72,827
Designated as Hedging Instrument [Member] \| Costless Collar [Member] \| Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	604	3,903
Designated as Hedging Instrument [Member] \| Costless Collar [Member] \| Long-Term Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	1,094	17,854
Not Designated as Hedging Instrument [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	782	33
Derivative Liability Not Designated as Hedging Instrument, Fair Value	2,915	2,008
Not Designated as Hedging Instrument [Member] \| Basis Swaps [Member] \| Hedging Asset [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	224	33
Not Designated as Hedging Instrument [Member] \| Basis Swaps [Member] \| Other Assets
Derivative Liability Not Designated as Hedging Instrument, Fair Value	558	0
Not Designated as Hedging Instrument [Member] \| Basis Swaps [Member] \| Hedging Liability [Member]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	1,934	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	$ 981	$ 0

Fair value as of the balance sheet date of the gross assets less the gross liabilities of a derivative asset or group of derivative assets.

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