Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 455.0	$ 487.5	$ 1,389.9	$ 1,423.9
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	329.8	322.7	999.2	949.7
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	113.8	129.1	349.8	344.9
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	11.4	10.3	28.8	46.1
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	0.0	7.7	3.7	32.7
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 0.0	$ 17.7	$ 8.4	$ 50.5