Financial Derivatives (Schedule of Gains and Losses on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	$ 82		$ 852
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(9,425)		(32,655)
Realized gains (losses) on financial derivatives, net	(9,343)	$ 781	(31,803)	$ (544)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	171		(60)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	1,318		(9,068)
Unrealized gains (losses) on financial derivatives, net	1,489	733	(9,128)	11,630
Foreign currency transactions		775		1,433
Foreign currency translation		(1,277)		(3,139)
Derivative [Member]
Derivative [Line Items]
Foreign currency transactions	17	0	47	(200)
Foreign currency translation	16	32	8	300
Interest rate swaps | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	82		852
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	34		(1,117)
Realized gains (losses) on financial derivatives, net	116	1,000	(265)	(500)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	171		(60)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(2,598)		(11,915)
Unrealized gains (losses) on financial derivatives, net	(2,427)	1,171	(11,975)	8,301
Credit default swaps on asset-backed securities | Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	16		419
Realized gains (losses) on financial derivatives, net	16	(986)	419	(739)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(12)		(394)
Unrealized gains (losses) on financial derivatives, net	(12)	1,307	(394)	759
Credit default swaps on asset-backed indices | Credit Risk [Member]
Derivative [Line Items]
Realized gains (losses) on financial derivatives, net		(153)		(2,199)
Unrealized gains (losses) on financial derivatives, net		(291)		1,161
Credit default swaps on corporate bond indices | Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(261)		(1,245)
Realized gains (losses) on financial derivatives, net	(261)		(1,245)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(81)		(832)
Unrealized gains (losses) on financial derivatives, net	(81)		(832)
Credit default swaps on corporate bond indices | Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(1,113)
Realized gains (losses) on financial derivatives, net	(1,113)	(1,085)		(3,322)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	909
Unrealized gains (losses) on financial derivatives, net	909	(247)		2,181
Credit default swaps on corporate bond indices | Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps			(4,387)
Realized gains (losses) on financial derivatives, net			(4,387)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps			(1,026)
Unrealized gains (losses) on financial derivatives, net			(1,026)
Credit default swaps on corporate bonds | Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(259)		(764)
Realized gains (losses) on financial derivatives, net	(259)	1,234	(764)	4,230
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	268		1,110
Unrealized gains (losses) on financial derivatives, net	268	(1,031)	1,110	(1,823)
Total return swaps | Equity Market/Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	251		(1,046)
Realized gains (losses) on financial derivatives, net	251	32	(1,046)	455
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(716)		(681)
Unrealized gains (losses) on financial derivatives, net	(716)	(1,425)	(681)	(1,739)
TBA securities | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(5,067)		(17,183)
Realized gains (losses) on financial derivatives, net	(5,067)		(17,183)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	1,542		3,680
Unrealized gains (losses) on financial derivatives, net	1,542		3,680
Futures | Interest Rate/Currency Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(3,551)		(8,365)
Realized gains (losses) on financial derivatives, net	(3,551)	729	(8,365)	1,652
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	1,627		761
Unrealized gains (losses) on financial derivatives, net	1,627	1,021	761	2,002
Forwards | Currency risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	525		1,068
Realized gains (losses) on financial derivatives, net	525	10	1,068	(59)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	379		228
Unrealized gains (losses) on financial derivatives, net	$ 379	228	228	712
Options | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps			(35)
Realized gains (losses) on financial derivatives, net			(35)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps			1
Unrealized gains (losses) on financial derivatives, net			$ 1
Options | Interest Rates/Equity Market Risk [Member]
Derivative [Line Items]
Realized gains (losses) on financial derivatives, net		0		(62)
Unrealized gains (losses) on financial derivatives, net		$ 0		$ 76