Financial Derivatives (Schedule of Gains and Losses on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	$ 143	$ 719
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(12,535)	(12,264)
Realized gains (losses) on financial derivatives, net	(12,392)	(11,545)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	(111)	(275)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(9,856)	(5,393)
Unrealized gains (losses) on financial derivatives, net	(9,967)	(5,668)
Derivative [Member]
Derivative [Line Items]
Foreign currency transactions	14	25
Foreign currency translation	17	21
Interest rate swaps | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	143	719
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(6,384)	1,458
Realized gains (losses) on financial derivatives, net	(6,241)	2,177
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	(111)	(275)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(20,020)	(5,774)
Unrealized gains (losses) on financial derivatives, net	(20,131)	(6,049)
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	(994)	275
Realized gains (losses) on financial derivatives, net	(994)	275
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	917	(239)
Unrealized gains (losses) on financial derivatives, net	917	(239)
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	(556)	(746)
Realized gains (losses) on financial derivatives, net	(556)	(746)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	12,524	(548)
Unrealized gains (losses) on financial derivatives, net	12,524	(548)
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	579	(2,513)
Realized gains (losses) on financial derivatives, net	579	(2,513)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	4,116	(2,407)
Unrealized gains (losses) on financial derivatives, net	4,116	(2,407)
Credit default swaps on corporate bonds | Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	94	(425)
Realized gains (losses) on financial derivatives, net	94	(425)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	1,527	766
Unrealized gains (losses) on financial derivatives, net	1,527	766
Total return swaps | Equity Market/Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(1,571)	(1,298)
Realized gains (losses) on financial derivatives, net	(1,571)	(1,298)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(213)	129
Unrealized gains (losses) on financial derivatives, net	(213)	129
TBA securities | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(3,173)	(6,435)
Realized gains (losses) on financial derivatives, net	(3,173)	(6,435)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(5,423)	1,898
Unrealized gains (losses) on financial derivatives, net	(5,423)	1,898
Futures | Interest Rate/Currency Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(1,122)	(2,433)
Realized gains (losses) on financial derivatives, net	(1,122)	(2,433)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(6,115)	359
Unrealized gains (losses) on financial derivatives, net	(6,115)	359
Forwards | Currency risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	592	(114)
Realized gains (losses) on financial derivatives, net	592	(114)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	253	423
Unrealized gains (losses) on financial derivatives, net	253	423
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	20
Unrealized gains (losses) on financial derivatives, net	20
Options | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps		(33)
Realized gains (losses) on financial derivatives, net		(33)
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
Options | Credit [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	2,558
Unrealized gains (losses) on financial derivatives, net	$ 2,558