Financial Derivatives (Schedule of Gains and Losses on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	$ 719
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(12,264)
Net Realized Gains (Losses) on Financial Derivatives	(11,545)	$ (1,503)
Unrealized gains (losses) on financial derivatives, net	(5,689)	4,044
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	(275)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(5,393)
Unrealized gains (losses) on financial derivatives, net	(5,668)
Foreign currency transactions	25	1,769
Foreign currency translation	21	101
Derivative [Member]
Derivative [Line Items]
Foreign currency transactions		(200)
Foreign currency translation		47
Interest rate swaps \| Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) on Periodic Settlements of Interest Rate Swaps	719
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	1,458
Net Realized Gains (Losses) on Financial Derivatives	2,177	(824)
Unrealized gains (losses) on financial derivatives, net		5,039
Change in Net Unrealized Gains (Losses) on Accrued Periodic Settlements of Interest Rate Swaps	(275)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(5,774)
Unrealized gains (losses) on financial derivatives, net	(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	275
Net Realized Gains (Losses) on Financial Derivatives	275	86
Unrealized gains (losses) on financial derivatives, net		(71)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(239)
Unrealized gains (losses) on financial derivatives, net	(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	(746)
Net Realized Gains (Losses) on Financial Derivatives	(746)	(1,562)
Unrealized gains (losses) on financial derivatives, net		1,563
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(548)
Unrealized gains (losses) on financial derivatives, net	(548)
Credit default swaps on asset-backed indices \| Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(2,513)
Net Realized Gains (Losses) on Financial Derivatives	(2,513)	(1,842)
Unrealized gains (losses) on financial derivatives, net		1,452
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	(2,407)
Unrealized gains (losses) on financial derivatives, net	(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	(425)
Net Realized Gains (Losses) on Financial Derivatives	(425)	4,469
Unrealized gains (losses) on financial derivatives, net		(3,855)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	766
Unrealized gains (losses) on financial derivatives, net	766
Total return swaps \| Equity Market/Credit Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(1,298)
Net Realized Gains (Losses) on Financial Derivatives	(1,298)	166
Unrealized gains (losses) on financial derivatives, net		17
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	129
Unrealized gains (losses) on financial derivatives, net	129
TBAs \| Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(6,435)
Net Realized Gains (Losses) on Financial Derivatives	(6,435)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	1,898
Unrealized gains (losses) on financial derivatives, net	1,898
Futures \| Interest Rate/Currency Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(2,433)
Net Realized Gains (Losses) on Financial Derivatives	(2,433)	(761)
Unrealized gains (losses) on financial derivatives, net		(561)
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	359
Unrealized gains (losses) on financial derivatives, net	359
Forwards \| Currency risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(114)
Net Realized Gains (Losses) on Financial Derivatives	(114)	(1,174)
Unrealized gains (losses) on financial derivatives, net		384
Change in Net Unrealized Gains (Losses) Other Than on Accrued Periodic Settlements of Interest Rate Swaps	423
Unrealized gains (losses) on financial derivatives, net	423
Options \| Interest Rate Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) Other Than Periodic Settlements of Interest Rate Swaps	(33)
Net Realized Gains (Losses) on Financial Derivatives	(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 \| Interest Rates/Equity Market Risk [Member]
Derivative [Line Items]
Net Realized Gains (Losses) on Financial Derivatives		(61)
Unrealized gains (losses) on financial derivatives, net		$ 76

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