Derivative Instruments (Gain (Loss) Recognized in the Consolidated Statements of Operations Related to its Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (66,348)	$ 4,070
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(63,321)	6,159
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Interest rate futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(28,787)	15,507
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	0	(901)
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Credit default swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(4,137)	255
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Equity futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(7,364)	0
Not Designated as Hedging Instrument \| Net foreign exchange gains (losses) \| Foreign currency forward contracts, underwriting and non-investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(21,142)	(3,641)
Not Designated as Hedging Instrument \| Net foreign exchange gains (losses) \| Foreign currency forward contracts, investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(1,891)	(5,061)
Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(3,027)	(2,089)
Designated as Hedging Instrument \| Foreign currency forward contracts, investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ (3,027)	$ (2,089)

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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Amount after tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669646-108580

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