Financial Derivatives (Schedule of Gains and Losses on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	$ (892)	$ 52	$ (750)	$ 770
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(10,771)	(10,966)	(23,306)	(23,229)
Realized gains (losses) on financial derivatives, net	(11,663)	(10,914)	(24,056)	(22,459)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	136	45	25	(231)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	8,040	(4,995)	(1,817)	(10,387)
Unrealized gains (losses) on financial derivatives, net	8,176	(4,950)	(1,792)	(10,618)
Derivative [Member]
Derivative [Line Items]
Foreign currency transactions	13	6	26	31
Foreign currency translation	3	(29)	19	(8)
Interest rate swaps | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	(892)	52	(750)	770
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(2,348)	(2,609)	(8,732)	(1,151)
Realized gains (losses) on financial derivatives, net	(3,240)	(2,557)	(9,482)	(381)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	136	45	25	(231)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	2,543	(3,545)	(17,477)	(9,317)
Unrealized gains (losses) on financial derivatives, net	2,679	(3,500)	(17,452)	(9,548)
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	38	129	(956)	403
Realized gains (losses) on financial derivatives, net	38	129	(956)	403
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(1)	(143)	916	(382)
Unrealized gains (losses) on financial derivatives, net	(1)	(143)	916	(382)
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	4,455	(239)	3,899	(984)
Realized gains (losses) on financial derivatives, net	4,455	(239)	3,899	(984)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(5,384)	(203)	7,139	(751)
Unrealized gains (losses) on financial derivatives, net	(5,384)	(203)	7,139	(751)
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	(62)	(761)	517	(3,274)
Realized gains (losses) on financial derivatives, net	(62)	(761)	517	(3,274)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	24	471	4,140	(1,936)
Unrealized gains (losses) on financial derivatives, net	24	471	4,140	(1,936)
Credit default swaps on corporate bonds | Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	354	(80)	448	(505)
Realized gains (losses) on financial derivatives, net	354	(80)	448	(505)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(482)	77	1,044	842
Unrealized gains (losses) on financial derivatives, net	(482)	77	1,044	842
Total return swaps | Equity Market/Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(484)	1	(2,055)	(1,297)
Realized gains (losses) on financial derivatives, net	(484)	1	(2,055)	(1,297)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	1,168	(94)	955	35
Unrealized gains (losses) on financial derivatives, net	1,168	(94)	955	35
TBA securities | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(5,972)	(5,681)	(9,145)	(12,115)
Realized gains (losses) on financial derivatives, net	(5,972)	(5,681)	(9,145)	(12,115)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	7,447	241	2,024	2,139
Unrealized gains (losses) on financial derivatives, net	7,447	241	2,024	2,139
Futures | Interest Rate/Currency Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(6,342)	(2,381)	(7,464)	(4,814)
Realized gains (losses) on financial derivatives, net	(6,342)	(2,381)	(7,464)	(4,814)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	5,629	(1,226)	(485)	(867)
Unrealized gains (losses) on financial derivatives, net	5,629	(1,226)	(485)	(867)
Forwards | Currency risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(310)	657	282	543
Realized gains (losses) on financial derivatives, net	(310)	657	282	543
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	56	(574)	309	(151)
Unrealized gains (losses) on financial derivatives, net	56	(574)	309	(151)
Warrants | Equity Market/Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	0		0
Realized gains (losses) on financial derivatives, net	0		0
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(402)		(382)
Unrealized gains (losses) on financial derivatives, net	(402)		(382)
Options | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps		(2)		(35)
Realized gains (losses) on financial derivatives, net		(2)		(35)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps		1		1
Unrealized gains (losses) on financial derivatives, net		$ 1		$ 1
Options | Credit [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(100)		(100)
Realized gains (losses) on financial derivatives, net	(100)		(100)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(2,558)		0
Unrealized gains (losses) on financial derivatives, net	$ (2,558)		$ 0