Financial Derivatives (Schedule of Gains and Losses on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	$ 52		$ 770
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(10,966)		(23,229)
Realized gains (losses) on financial derivatives, net	(10,914)	$ 177	(22,459)	$ (1,325)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	(45)		231
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	4,995		10,387
Unrealized gains (losses) on financial derivatives, net	(4,950)	6,855	(10,618)	10,899
Foreign currency transactions		(1,110)		658
Foreign currency translation		(1,964)		(1,863)
Derivative [Member]
Derivative [Line Items]
Foreign currency transactions	6	0	31	(200)
Foreign currency translation	(29)	200	(8)	300
Interest rate swaps | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	52		770
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(2,609)		(1,151)
Realized gains (losses) on financial derivatives, net	(2,557)	(677)	(381)	(1,500)
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	(45)		231
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	3,545		9,317
Unrealized gains (losses) on financial derivatives, net	(3,500)	2,091	(9,548)	7,130
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	129		403
Realized gains (losses) on financial derivatives, net	129	161	403	247
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	143		382
Unrealized gains (losses) on financial derivatives, net	(143)	(477)	(382)	(549)
Credit default swaps on asset-backed indices | Credit Risk [Member]
Derivative [Line Items]
Realized gains (losses) on financial derivatives, net		(204)		(2,046)
Unrealized gains (losses) on financial derivatives, net		1		1,453
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)		(984)
Realized gains (losses) on financial derivatives, net	(239)		(984)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	203		751
Unrealized gains (losses) on financial derivatives, net	(203)		(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	(761)
Realized gains (losses) on financial derivatives, net	(761)	(675)		(2,237)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(471)
Unrealized gains (losses) on financial derivatives, net	471	866		2,428
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			(3,274)
Realized gains (losses) on financial derivatives, net			(3,274)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps			1,936
Unrealized gains (losses) on financial derivatives, net			(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	(80)		(505)
Realized gains (losses) on financial derivatives, net	(80)	(1,472)	(505)	2,996
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(77)		(842)
Unrealized gains (losses) on financial derivatives, net	77	3,063	842	(792)
Total return swaps | Equity Market/Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	1		(1,297)
Realized gains (losses) on financial derivatives, net	1	257	(1,297)	423
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	94		(35)
Unrealized gains (losses) on financial derivatives, net	(94)	(331)	35	(313)
TBA securities | Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(5,681)		(12,115)
Realized gains (losses) on financial derivatives, net	(5,681)		(12,115)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(241)		(2,139)
Unrealized gains (losses) on financial derivatives, net	241		2,139
Futures | Interest Rate/Currency Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(2,381)		(4,814)
Realized gains (losses) on financial derivatives, net	(2,381)	1,684	(4,814)	923
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	1,226		867
Unrealized gains (losses) on financial derivatives, net	(1,226)	1,542	(867)	981
Forwards | Currency risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	657		543
Realized gains (losses) on financial derivatives, net	657	1,104	543	(69)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	574		151
Unrealized gains (losses) on financial derivatives, net	(574)	100	(151)	485
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 | Interest Rates/Equity Market Risk [Member]
Derivative [Line Items]
Realized gains (losses) on financial derivatives, net		(1)		(62)
Unrealized gains (losses) on financial derivatives, net		$ 0		$ 76