Derivatives and Risk Management (Balance Sheet Classification) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative assets	$ 306,728	$ 292,144
Derivative liabilities	42,527	4,266
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	201,755	287,993
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative assets	189,541	279,443
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative assets	12,214	8,550
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	104,973	4,151
Derivative liabilities	42,527	4,266
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative assets	48,512
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative assets	39,276
Not Designated as Hedging Instrument [Member] | Fixed Price Call Options [Member] | Long-Term Hedging Liability [Member]
Derivative [Line Items]
Derivative liabilities	38,856	4,128
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative assets	8,402	3,250
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative assets	2,565	901
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative liabilities	69	138
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative assets	6,218
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative liabilities	308
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Long-Term Hedging Liability [Member]
Derivative [Line Items]
Derivative liabilities	$ 3,294