Investment Information - Investment in operating affiliates (Details) € / shares in Units, $ / shares in Units, € in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
	Jul. 01, 2021 USD ($) $ / shares	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 EUR (€)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 € / shares	Dec. 31, 2020 USD ($)
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates				$ 753,916		$ 0
Investment in operating affiliates		$ 1,111,825		1,111,825				$ 129,291
Income (loss) from operating affiliates		124,119	$ 919	224,052		$ 6,262
Net loss on deconsolidation		33,100		33,100
Watford Holdings Ltd | Merger Agreement
Schedule of Equity Method Investments [Line Items]
Business acquisition, share price (per share) | $ / shares	$ 35.00
Business Acquisition Consideration Paid By Acquirer	$ 278,900
Gain on merger transaction	$ 95,700
Greysbridge
Schedule of Equity Method Investments [Line Items]
Percentage ownership after merger transaction	40.00%
Watford Merger Transaction | Watford Holdings Ltd
Schedule of Equity Method Investments [Line Items]
Net loss on deconsolidation		$ 33,100		$ 33,100
Kelso And Company | Greysbridge
Schedule of Equity Method Investments [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	30.00%
Warburg Pincus LLC | Greysbridge
Schedule of Equity Method Investments [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	30.00%
Coface
Schedule of Equity Method Investments [Line Items]
Percentage ownership		29.50%		29.50%
Consideration paid per share | € / shares							€ 9.95
Payments to Acquire Interest in Subsidiaries and Affiliates				$ 546,000	€ 453,000
Investment in operating affiliates		$ 615,900		615,900
Income (loss) from operating affiliates				74,500
Purchase Of Affiliate, Deferred Gain		$ 36,000		$ 36,000
Purchase Of Operating Affiliate, Deferred Gain, Period Of Recognition				5 years	5 years
Coface | Issued Common Stock
Schedule of Equity Method Investments [Line Items]
Percentage ownership		29.86%		29.86%
Coface | Outstanding Common Stock
Schedule of Equity Method Investments [Line Items]
Percentage ownership		30.10%		30.10%
Greysbridge
Schedule of Equity Method Investments [Line Items]
Investment in operating affiliates		$ 363,300		$ 363,300