Derivatives and Risk Management (Balance Sheet Classification) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Total derivative assets	$ 167,361	$ 292,144
Total derivative liabilities	62,807	4,266
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	135,469	287,993
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	131,091	279,443
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	4,378	8,550
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	31,892	4,151
Derivative Liability Not Designated as Hedging Instrument, Fair Value	62,807	4,266
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	985
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	29,507
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	436
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Long-Term Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	824
Not Designated as Hedging Instrument [Member] | Fixed Price Call Options [Member] | Long-Term Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	61,210	4,128
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	936	3,250
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	464	901
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	$ 337	$ 138