Commitments and Contingencies (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2020 GBP (£)
Other Commitments [Line Items]
Operating expense	$ 34,773	$ 33,290
Data analytics and services
Other Commitments [Line Items]
Purchase commitment, period (in years)	2 years
Annual fee	$ 4,800
Operating expense	1,200	$ 1,000
Lloyd's Syndicates
Other Commitments [Line Items]
Current lending capacity under revolving credit agreement | £				£ 30,000,000.0
Unused commitments to extend credit	$ 37,300
Non-renew notice, period	30 days
Interest rate on revolving credit agreement (percent)	3.80%			3.80%
Required FAL deposit	$ 136,200
Funding Commitments
Other Commitments [Line Items]
Commitments total	223,500
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	2,400
Maximum | Data analytics and services
Other Commitments [Line Items]
Purchase obligation	$ 3,200