Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		$ 5,941,000	$ 5,880,735	$ 4,041,076	$ 4,500,802
Reinsurance recoverable | Reinsurer concentration risk | AM Best A minus Or Better Rating
Ceded Credit Risk [Line Items]
Concentration risk percentage		69.50%	69.70%
Reinsurance recoverable | Reinsurer concentration risk | AM Best Rating Below A Minus | All Other Carriers
Ceded Credit Risk [Line Items]
Concentration risk percentage		0.10%	0.10%
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | All Other Carriers
Ceded Credit Risk [Line Items]
Concentration risk percentage	[1]	30.40%	30.20%
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | Reinsurance trusts or letters of credit
Ceded Credit Risk [Line Items]
Concentration risk percentage		93.00%	91.00%
Stockholders' equity | Reinsurer concentration risk | Largest Balance Due From Any One Carrier
Ceded Credit Risk [Line Items]
Concentration risk percentage		7.20%	6.70%

[1] At March 31, 2022 and December 31, 2021 over 93% and 91% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively