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Investment Information - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) lots	Dec. 31, 2019 USD ($) lots	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Investable assets	$ 29,513,907	$ 24,990,265	$ 22,324,524
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots	2,320	2,230
Total number of positions (lots) | lots	11,180	9,590
Largest single loss	$ 900	$ 900
Equity securities, at fair value	1,444,830	838,925
Equity Method Investment, Financial Statement, Reported Amounts [Abstract]
Equity in net income (loss) of investment funds accounted for using the equity method	$ 146,693	$ 123,672	$ 45,641
Equity method investments, time lag for reporting - low end of range	one
Equity method investments, time lag for reporting - high end of range	three
Watford Holdings Ltd
Segment Reporting Information [Line Items]
Investable assets	$ 2,700,000
ACGL
Segment Reporting Information [Line Items]
Investable assets	$ 26,900,000

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