Commitments and Contingencies (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 GBP (£)
Other Commitments [Line Items]
Net investment income	$ 18,124	$ 23,539	$ 38,954	$ 46,357
Operating expense	33,349	34,661	$ 68,120	67,951
Data analytics and services
Other Commitments [Line Items]
Purchase commitment, period (in years)			2 years
Annual fee	4,800		$ 4,800
Operating expense	1,200	$ 1,300	2,400	$ 2,300
Lloyd's Syndicates
Other Commitments [Line Items]
Current lending capacity under revolving credit agreement | £						£ 30,000,000.0
Unused commitments to extend credit	$ 37,200		$ 37,200
Non-renew notice, period			30 days
Interest rate on revolving credit agreement (percent)	3.80%		3.80%			3.80%
Required FAL deposit	$ 137,500		$ 137,500
Funding Commitments
Other Commitments [Line Items]
Commitments total	213,300		213,300
NORCAL Group | Forecast
Other Commitments [Line Items]
Base consideration					$ 450,000
Contingent consideration (up to)					$ 150,000
Minimum | Data analytics and services
Other Commitments [Line Items]
Purchase obligation	1,200		1,200
Maximum | Data analytics and services
Other Commitments [Line Items]
Purchase obligation	$ 1,600		$ 1,600