Derivative Financial Instruments Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Offsetting Assets and Liabilities [Line Items]
Notional Amount	$ 1,581,951		$ 1,403,512
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(13,141)		(43,048)
Cash Collateral Posted	16,165		42,770
Dealer A [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Amount	383,632		561,716
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(7,162)		(17,507)
Cash Collateral Posted	7,100		17,900
Net Exposure	0	[1]	393	[1]
Dealer B [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Amount	343,467		403,097
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(6,887)		(15,784)
Cash Collateral Posted	7,100		16,980
Net Exposure	213	[1]	1,196	[1]
Dealer C [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Amount	14,666		15,221
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(1,491)		(2,037)
Cash Collateral Posted	0		0
Net Exposure	0	[1]	0	[1]
Dealer D [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Amount	291,320		184,648
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	3,911		(2,453)
Cash Collateral Posted			2,600
Cash Collateral (Received)	(4,100)
Net Exposure	0	[1]	147	[1]
Dealer E [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Amount	283,681		238,830
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	942		(5,267)
Cash Collateral Posted			5,290
Cash Collateral (Received)	(1,160)
Net Exposure	0	[1]	23	[1]
Dealer F [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Amount	265,185
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(2,454)
Cash Collateral Posted	7,225
Net Exposure	4,771	[1]
Designated as Hedging Instrument [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	3,047		0
MTM Loss	(2,194)		(1,627)
Designated as Hedging Instrument [Member] | Dealer A [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	0		0
MTM Loss	(388)		(985)
Designated as Hedging Instrument [Member] | Dealer B [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	0		0
MTM Loss	0		(642)
Designated as Hedging Instrument [Member] | Dealer C [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	0		0
MTM Loss	0		0
Designated as Hedging Instrument [Member] | Dealer D [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	1,471		0
MTM Loss	0		0
Designated as Hedging Instrument [Member] | Dealer E [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	1,576		0
MTM Loss	(513)		0
Designated as Hedging Instrument [Member] | Dealer F [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	0
MTM Loss	(1,293)
Non-Hedge Accounting [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	12,846		544
MTM Loss	(26,840)		(41,965)
Non-Hedge Accounting [Member] | Dealer A [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	4,009		199
MTM Loss	(10,783)		(16,721)
Non-Hedge Accounting [Member] | Dealer B [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	3,241		139
MTM Loss	(10,128)		(15,281)
Non-Hedge Accounting [Member] | Dealer C [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	1		1
MTM Loss	(1,492)		(2,038)
Non-Hedge Accounting [Member] | Dealer D [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	3,150		53
MTM Loss	(710)		(2,506)
Non-Hedge Accounting [Member] | Dealer E [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	2,373		152
MTM Loss	(2,494)		(5,419)
Non-Hedge Accounting [Member] | Dealer F [Member]
Offsetting Assets and Liabilities [Line Items]
MTM Gain	72
MTM Loss	$ (1,233)

[1] Net positive exposure represents over-collateralized loss positions which can be the result of OTC clearing house initial margin requirements posted in compliance with the Dodd-Frank Wall Street Reform and Consumer Protection Act ("Dodd-Frank").