Derivative Financial Instruments (Schedule of offsetting financial assets and liabilities for non-customer derivative positions) (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Notional Amount		$ 5,514,013	$ 10,327,833
Dealer A [Member]
Offsetting Liabilities [Line Items]
Notional Amount		391,745	427,430
(Liability)/ Asset, net		(4,046)	(5,454)
Cash Collateral Posted		3,800	5,300
Net Exposure	[1]	0	0
Dealer B [Member]
Offsetting Liabilities [Line Items]
Notional Amount		274,353	319,663
(Liability)/ Asset, net		(3,004)	(3,948)
Cash Collateral Posted		3,020	3,610
Net Exposure	[1]	16	0
Dealer C [Member]
Offsetting Liabilities [Line Items]
Notional Amount		5,775	11,538
(Liability)/ Asset, net		(433)	(834)
Cash Collateral Posted		0	0
Net Exposure	[1]	0	0
Dealer D [Member]
Offsetting Liabilities [Line Items]
Notional Amount		387,550	303,663
(Liability)/ Asset, net		(1,546)
Cash Collateral Posted		1,300
(Liability)/ Asset, net			267
Cash Collateral Posted (Received)			(400)
Net Exposure	[1]	0	0
Dealer E [Member]
Offsetting Liabilities [Line Items]
Notional Amount		421,545	424,401
(Liability)/ Asset, net		(644)
Cash Collateral Posted		470
(Liability)/ Asset, net			1,409
Cash Collateral Posted (Received)			(1,420)
Net Exposure	[1]	0	0
DCO [Member]
Offsetting Liabilities [Line Items]
Notional Amount		1,352,206	6,631,936
(Liability)/ Asset, net		(27,226)	(20,932)
Cash Collateral Posted		42,975	39,037
Net Exposure	[1]	15,749	18,105
Dealers Total [Member]
Offsetting Liabilities [Line Items]
Notional Amount		2,833,174	8,118,631
(Liability)/ Asset, net		(36,899)	(29,492)
Cash Collateral Posted		51,565	46,127
Designated as Hedging Instrument [Member] | Dealer A [Member]
Offsetting Liabilities [Line Items]
Gain		248	0
Loss		(1,359)	(739)
Designated as Hedging Instrument [Member] | Dealer B [Member]
Offsetting Liabilities [Line Items]
Gain		752	1,494
Loss		0	0
Designated as Hedging Instrument [Member] | Dealer C [Member]
Offsetting Liabilities [Line Items]
Gain		0	0
Loss		0	0
Designated as Hedging Instrument [Member] | Dealer D [Member]
Offsetting Liabilities [Line Items]
Gain		377	747
Loss		0	0
Designated as Hedging Instrument [Member] | Dealer E [Member]
Offsetting Liabilities [Line Items]
Gain		1,130	2,240
Loss		0	0
Designated as Hedging Instrument [Member] | DCO [Member]
Offsetting Liabilities [Line Items]
Gain		0	0
Loss		0	(3,858)
Designated as Hedging Instrument [Member] | Dealers Total [Member]
Offsetting Liabilities [Line Items]
Gain		2,507	4,481
Loss		(1,359)	(4,597)
Non-Hedge Accounting [Member] | Dealer A [Member]
Offsetting Liabilities [Line Items]
Gain		504	1,861
Loss		(3,439)	(6,576)
Non-Hedge Accounting [Member] | Dealer B [Member]
Offsetting Liabilities [Line Items]
Gain		348	978
Loss		(4,104)	(6,420)
Non-Hedge Accounting [Member] | Dealer C [Member]
Offsetting Liabilities [Line Items]
Gain		0	0
Loss		(433)	(834)
Non-Hedge Accounting [Member] | Dealer D [Member]
Offsetting Liabilities [Line Items]
Gain		125	1,147
Loss		(2,048)	(1,627)
Non-Hedge Accounting [Member] | Dealer E [Member]
Offsetting Liabilities [Line Items]
Gain		138	867
Loss		(1,912)	(1,698)
Non-Hedge Accounting [Member] | DCO [Member]
Offsetting Liabilities [Line Items]
Gain		1,140	555
Loss		(28,366)	(17,629)
Non-Hedge Accounting [Member] | Dealers Total [Member]
Offsetting Liabilities [Line Items]
Gain		2,255	5,408
Loss		$ (40,302)	$ (34,784)

[1] Net positive exposure represents over-collateralized loss positions which can result from DCO initial margin requirements posted in compliance with the Dodd-Frank Wall Street Reform and Consumer Protection Act.