Investment Information - Net realized gains and losses (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Net realized gains and losses:
Available for sale securities, gross gains on investment sales		$ 81,161	$ 313,886	$ 595,941
Available for sale securities, gross losses on investment sales		(317,150)	(156,791)	(117,282)
Equity securities at fair value, net realized gains (losses) on sales during the period		74,922	122,606	26,849
Equity securities at fair value, net unrealized gains (losses) on equity securities still held at reporting date		(267,927)	48,746	102,394
Allowance for credit losses - investment related		(43,909)	(2,100)	(3,597)
Allowance for credit losses - underwriting related		(13,274)	1,062	(10,007)
Derivative instruments		(75,023)	(32,390)	179,675
Other	[1]	(3,553)	(53,537)	3,575
Net realized gains (losses)		$ (662,734)	379,845	823,460
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]		Net realized gains (losses)
Somers Holdings Ltd.
Net realized gains and losses:
Gain (loss) on deconsolidation			(33,100)
Fixed maturities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		$ (70,951)	7,953	15,881
Other investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		(19,498)	116,781	13,656
Equity securities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		(4,520)	13,028	14,629
Short-term investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		$ (3,012)	$ 601	$ 2,279

[1] 2021 period reflected $33.1 million of losses related to the Company’sdeconsolidation of Somers