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Investment Information - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) lots	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) lots	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) lots
Narrative items:
Investable assets	$ 28,420,000		$ 28,420,000
Equity in net (loss) income of investment funds accounted for using the equity method	$ 126,735	$ 17,130	$ 57,407	$ 96,533
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots	3,270		3,270		2,230
Total number of positions (lots) | lots	11,090		11,090		9,590
Largest single loss	$ 900		$ 900		$ 900
Equity securities, at fair value	1,502,015		$ 1,502,015		$ 838,925
Minimum
Narrative items:
Time lag for reporting			one
Maximum
Narrative items:
Time lag for reporting			three
Underwriting segments
Narrative items:
Investable assets	25,720,000		$ 25,720,000
Equity in net (loss) income of investment funds accounted for using the equity method	126,735	17,130	57,407	96,533
Other
Narrative items:
Investable assets	2,690,000		2,690,000
Equity in net (loss) income of investment funds accounted for using the equity method	$ 0	$ 0	$ 0	$ 0

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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