Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 454.3	$ 470.3	$ 934.9	$ 936.4
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	333.0	314.2	669.4	627.0
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	109.0	116.8	236.1	215.8
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	11.0	10.7	17.4	35.8
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	1.3	11.0	3.6	25.0
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 0.0	$ 17.6	$ 8.4	$ 32.8