Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 1,910.1	$ 1,780.5	$ 1,729.7
United States (Federal)
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	1,277.0	1,205.0	1,115.8
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	475.3	361.1	391.5
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	54.8	96.5	80.7
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	35.4	59.5	85.0
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 67.6	$ 58.4	$ 56.7