Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2021	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and LAE		$ 4,041,076	$ 4,500,802
Reinsurance recoverable | Credit concentration risk | AM Best A minus Or Better Rating
Ceded Credit Risk [Line Items]
Concentration risk percentage		64.50%	63.90%
Reinsurance recoverable | Credit concentration risk | Not Rated
Ceded Credit Risk [Line Items]
Concentration risk percentage	[1]	35.50%	36.10%
Reinsurance recoverable | Credit concentration risk | Not Rated | Reinsurance trusts or letters of credit
Ceded Credit Risk [Line Items]
Concentration risk percentage		92.00%	94.00%
Stockholders' equity | Reinsurer concentration risk
Ceded Credit Risk [Line Items]
Concentration risk percentage		1.70%	1.80%

[1] At March 31, 2021 and December 31, 2020 over 92% and 94% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively