Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 1,740.4	$ 1,910.1	$ 1,780.5
United States (Federal)
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	1,209.9	1,277.0	1,205.0
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	480.2	475.3	361.1
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	38.0	54.8	96.5
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	3.9	35.4	59.5
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 8.4	$ 67.6	$ 58.4