Reinsurance - Ceded credit risk (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		$ 4,346,816	$ 2,919,372
Concentration risk percentage		10.00%
Credit concentration risk | Reinsurance recoverable | Unrated fully collateralized reinsurers
Concentration Risk [Line Items]
Concentration risk percentage	[1]	13.50%	12.90%
Reinsurer concentration risk | Stockholders' equity
Concentration Risk [Line Items]
Concentration risk percentage		1.70%	2.70%
AM Best A minus Or Better Rating | Credit concentration risk | Reinsurance recoverable
Concentration Risk [Line Items]
Concentration risk percentage		61.20%	63.00%
Not Rated | Credit concentration risk | Reinsurance recoverable
Concentration Risk [Line Items]
Concentration risk percentage	[2]	25.30%	24.10%
Reinsurance trusts or letters of credit | Not Rated | Credit concentration risk | Reinsurance recoverable
Concentration Risk [Line Items]
Concentration risk percentage		90.00%

[1] Such amount is fully collateralized through reinsurance trusts

[2] Over 90% of such amount is collateralized through reinsurance trusts or letters of credit.