Derivative Instruments (Schedule of Gain (Loss) Recognized in Consolidated Statements of Operations Related to Principal Derivative Instruments) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (37,444)	$ 100,152	$ 70,862
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(32,909)	88,467	69,903
Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(4,535)	11,685	959
Foreign currency forward contracts \| Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated other comprehensive income (loss)	(4,535)	11,685	959
Net realized and unrealized (losses) gains on investments \| Interest rate futures \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(15,846)	103,102	16,848
Net realized and unrealized (losses) gains on investments \| Interest rate swaps \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(1,184)	2,334	1,488
Net realized and unrealized (losses) gains on investments \| Credit default swaps \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	3,479	(1,304)	7,043
Net realized and unrealized (losses) gains on investments \| Total return swaps \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	1,314	(5,479)	12,155
Net realized and unrealized (losses) gains on investments \| Equity futures \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	0	(30,045)	21,357
Net foreign exchange (losses) gains \| Foreign currency forward contracts \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(19,151)	24,309	12,617
Net foreign exchange (losses) gains \| Foreign currency forward contracts \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (1,521)	$ (4,450)	$ (1,605)

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
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