Derivatives and Risk Mangement (Balance Sheet Classification) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Total derivative assets	$ 717,343	$ 208,325
Total derivative liabilities	12,513	47,873
Designated as Hedging Instrument [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	713,640	208,292
Derivative Liability Designated as Hedging Instrument, Fair Value	12,058	45,865
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	333,479	81,797
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Other Assets
Derivative Asset Designated as Hedging Instrument, Fair Value	201,081	5,086
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	11,849	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	0	22,334
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	179,080	48,582
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Other Assets
Derivative Asset Designated as Hedging Instrument, Fair Value	0	72,827
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	209	3,903
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Long-Term Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	0	17,854
Not Designated as Hedging Instrument [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	3,703	33
Derivative Liability Not Designated as Hedging Instrument, Fair Value	455	2,008
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Asset [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	1,906	33
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Assets
Derivative Asset Not Designated as Hedging Instrument, Fair Value	1,797	0
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Liability [Member]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	400	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	$ 55	$ 0