Investment Information - Net realized gains and losses (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net realized gains and losses:
Available for sale securities, gross gains on investment sales		$ 313,886	$ 595,941	$ 235,655
Available for sale securities, gross losses on investment sales		(156,791)	(117,282)	(104,612)
Equity securities at fair value, net realized gains (losses) on sales during the period		122,606	26,849	11,313
Equity securities at fair value, net unrealized gains (losses) on equity securities still held at reporting date		48,746	102,394	97,768
Allowance for credit losses - investment related		(2,100)	(3,597)	0
Allowance for credit losses - underwriting related		1,062	(10,007)	0
Net impairment losses		0	533
Net impairment losses				(3,165)
Derivative instruments		(32,390)	179,675	119,741
Other	[1]	(53,537)	3,575	(19,348)
Net realized gains (losses)		379,845	823,460	363,198
Greysbridge 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		7,953	15,881	41,910
Other investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		116,781	13,656	(35,734)
Equity securities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		13,028	14,629	15,869
Short-term investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		$ 601	$ 2,279	$ 3,801

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