Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 389.9	$ 480.6
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	325.6	336.4
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	48.3	127.1
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	13.2	6.4
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	0.0	2.3
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 2.8	$ 8.4