Commitments and Contingencies (Details)		3 Months Ended	6 Months Ended
	May 05, 2021 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)
Other Commitments [Line Items]
Contingent consideration		$ 24,000,000	$ 24,000,000		$ 0	$ 0
NORCAL Group
Other Commitments [Line Items]
Percentage of interest acquired	98.00%
Total acquisition consideration	$ 448,809,000
Contingent consideration, maximum estimate	84,000,000.0
Contingent consideration	$ 24,000,000.0	24,000,000.0	24,000,000.0
Funding Commitments
Other Commitments [Line Items]
Commitments total		229,400,000	229,400,000
Lloyd's Syndicates
Other Commitments [Line Items]
Current lending capacity under revolving credit agreement | £				£ 30,000,000
Unused commitments to extend credit		$ 41,500,000	$ 41,500,000
Non-renew notice, period			30 days
Auto-renewal period prior to maturity date			1 year
Interest rate on revolving credit agreement (percent)		3.80%	3.80%	3.80%
Outstanding borrowings		$ 0	$ 0
Required FAL deposit		72,300,000	$ 72,300,000
Syndicate 1729
Other Commitments [Line Items]
Return of deposit assets		$ 24,500,000