Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 480.6	$ 466.1
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	336.4	312.8
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	127.1	99.0
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	6.4	25.1
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	2.3	14.0
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 8.4	$ 15.2