Derivative Financial Instruments Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Offsetting Assets and Liabilities [Line Items]
Notional Outstanding	$ 1,340,819		$ 1,403,512
Total Gain (Loss)	(7,611)		(43,048)
Cash Collateral	4,283		42,770
Dealer A [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Outstanding	367,672		561,716
Total Gain (Loss)	(6,328)		(17,507)
Cash Collateral	5,800		17,900
Net Exposure	0	[1]	393	[1]
Dealer B [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Outstanding	346,099		403,097
Total Gain (Loss)	(6,330)		(15,784)
Cash Collateral	5,160		16,980
Net Exposure	0	[1]	1,196	[1]
Dealer C [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Outstanding	14,853		15,221
Total Gain (Loss)	(1,575)		(2,037)
Cash Collateral	0		0
Net Exposure	0	[1]	0	[1]
Dealer D [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Outstanding	298,038		184,648
Total Gain (Loss)	4,830		(2,453)
Cash Collateral	(5,200)		2,600
Net Exposure	0	[1]	147	[1]
Dealer E [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Outstanding	285,445		238,830
Total Gain (Loss)	1,690		(5,267)
Cash Collateral	(1,830)		5,290
Net Exposure	0	[1]	23	[1]
Dealer F [Member]
Offsetting Assets and Liabilities [Line Items]
Notional Outstanding	28,712
Total Gain (Loss)	102
Cash Collateral	353
Net Exposure	455
Hedge Accounting [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	3,056	[2]	0	[2]
Loss	0	[2]	1,627	[2]
Hedge Accounting [Member] | Dealer A [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	0	[2]	0	[2]
Loss	0	[2]	(985)	[2]
Hedge Accounting [Member] | Dealer B [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	0	[2]	0	[2]
Loss	0	[2]	(642)	[2]
Hedge Accounting [Member] | Dealer C [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	0	[2]	0	[2]
Loss	0	[2]	0	[2]
Hedge Accounting [Member] | Dealer D [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	1,470	[2]	0	[2]
Loss	0	[2]	0	[2]
Hedge Accounting [Member] | Dealer E [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	1,586	[2]	0	[2]
Loss	0	[2]	0	[2]
Hedge Accounting [Member] | Dealer F [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	0
Loss	0
Non-Hedge Accounting [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	15,412	[3]	544	[3]
Loss	(26,079)	[3]	(41,965)	[3]
Total Gain (Loss)	(10,667)	[3]	(41,421)	[3]
Non-Hedge Accounting [Member] | Dealer A [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	4,797	[3]	199	[3]
Loss	(11,125)	[3]	(16,721)	[3]
Total Gain (Loss)	(6,328)	[3]	(16,522)	[3]
Non-Hedge Accounting [Member] | Dealer B [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	3,875	[3]	139	[3]
Loss	(10,205)	[3]	(15,281)	[3]
Total Gain (Loss)	(6,330)	[3]	(15,142)	[3]
Non-Hedge Accounting [Member] | Dealer C [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	1	[3]	1	[3]
Loss	(1,576)	[3]	(2,038)	[3]
Total Gain (Loss)	(1,575)	[3]	(2,037)	[3]
Non-Hedge Accounting [Member] | Dealer D [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	3,922	[3]	53	[3]
Loss	(562)	[3]	(2,506)	[3]
Total Gain (Loss)	3,360	[3]	(2,453)	[3]
Non-Hedge Accounting [Member] | Dealer E [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	2,715	[3]	152	[3]
Loss	(2,611)	[3]	(5,419)	[3]
Total Gain (Loss)	104	[3]	(5,267)	[3]
Non-Hedge Accounting [Member] | Dealer F [Member]
Offsetting Assets and Liabilities [Line Items]
Gain	102
Loss	0
Total Gain (Loss)	$ 102

[1] Net positive exposure represents over collateralized loss positions which are the result of OTC clearing house initial margin requirements posted in compliance with Dodd-Frank regulations.

[2] Hedge accounting positions are recorded on a gross basis in other assets (liabilities)

[3] Net gain (loss) position recorded in other assets (liabilities)