Transactions With Related Parties (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Jan. 01, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	$ 5,358,852	$ 4,621,937	$ 5,358,852	$ 4,621,937	$ 4,314,515		$ 4,500,802	$ 4,363,507	$ 4,346,816
Ceded Premiums Written	1,131,486	$ 806,888	2,907,002	$ 2,151,853
Reinsurance balances payable	1,403,929		1,403,929				683,263
Operating affilicates | Watford Holdings Ltd
Related Party Transaction [Line Items]
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	874,900		874,900
Senior notes, face amount	$ 35,000		$ 35,000
Senior notes, stated percentage	6.50%		6.50%
Ceded Premiums Written	$ 316,200
Reinsurance balances payable	281,200		$ 281,200
Operating affilicates | Watford Holdings Ltd | Non-cumulative preferred shares
Related Party Transaction [Line Items]
Ownership percentage			6.60%
Operating affilicates | Premia Holdings Ltd
Related Party Transaction [Line Items]
Legacy business, amount of liabilities transferred						$ 380,000
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0		$ 0				199,800
Funds held liability	$ 8,800		$ 8,800				$ 149,600