Financial Derivatives (Schedule of Gains and Losses on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	$ (1,150)	$ 82	$ (1,900)	$ 852
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(485)	(9,425)	(23,790)	(32,655)
Realized gains (losses) on financial derivatives, net	(1,635)	(9,343)	(25,690)	(31,803)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	516	171	541	(60)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(794)	1,318	(2,611)	(9,068)
Unrealized gains (losses) on financial derivatives, net	(278)	1,489	(2,070)	(9,128)
Derivative [Member]
Derivative [Line Items]
Foreign currency transactions	(15)	17	12	47
Foreign currency translation	20	16	39	8
Interest rate swaps | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	(1,150)	82	(1,900)	852
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(29)	34	(8,761)	(1,117)
Realized gains (losses) on financial derivatives, net	(1,179)	116	(10,661)	(265)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	516	171	541	(60)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	1,104	(2,598)	(16,373)	(11,915)
Unrealized gains (losses) on financial derivatives, net	1,620	(2,427)	(15,832)	(11,975)
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	1	16	(955)	419
Realized gains (losses) on financial derivatives, net	1	16	(955)	419
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(1)	(12)	915	(394)
Unrealized gains (losses) on financial derivatives, net	(1)	(12)	915	(394)
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	(239)	(261)	3,660	(1,245)
Realized gains (losses) on financial derivatives, net	(239)	(261)	3,660	(1,245)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	503	(81)	7,642	(832)
Unrealized gains (losses) on financial derivatives, net	503	(81)	7,642	(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	(1,067)	(1,113)	(550)	(4,387)
Realized gains (losses) on financial derivatives, net	(1,067)	(1,113)	(550)	(4,387)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(1,059)	909	3,080	(1,026)
Unrealized gains (losses) on financial derivatives, net	(1,059)	909	3,080	(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	(93)	(259)	356	(764)
Realized gains (losses) on financial derivatives, net	(93)	(259)	356	(764)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(334)	268	711	1,110
Unrealized gains (losses) on financial derivatives, net	(334)	268	711	1,110
Total return swaps | Equity Market/Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(125)	251	(2,180)	(1,046)
Realized gains (losses) on financial derivatives, net	(125)	251	(2,180)	(1,046)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(348)	(716)	607	(681)
Unrealized gains (losses) on financial derivatives, net	(348)	(716)	607	(681)
TBA securities | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	2,367	(5,067)	(6,778)	(17,183)
Realized gains (losses) on financial derivatives, net	2,367	(5,067)	(6,778)	(17,183)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(744)	1,542	1,280	3,680
Unrealized gains (losses) on financial derivatives, net	(744)	1,542	1,280	3,680
Futures | Interest Rate/Currency Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(402)	(3,551)	(7,866)	(8,365)
Realized gains (losses) on financial derivatives, net	(402)	(3,551)	(7,866)	(8,365)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	52	1,627	(434)	761
Unrealized gains (losses) on financial derivatives, net	52	1,627	(434)	761
Forwards | Currency risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(898)	525	(616)	1,068
Realized gains (losses) on financial derivatives, net	(898)	525	(616)	1,068
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	33	379	343	228
Unrealized gains (losses) on financial derivatives, net	$ 33	$ 379	343	228
Warrants | Equity Market/Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps			0
Realized gains (losses) on financial derivatives, net			0
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps			(382)
Unrealized gains (losses) on financial derivatives, net			(382)
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 | Credit [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps			(100)
Realized gains (losses) on financial derivatives, net			(100)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps			0
Unrealized gains (losses) on financial derivatives, net			$ 0