Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 487.5	$ 445.5	$ 1,423.9	$ 1,313.9
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	322.7	299.1	949.7	900.2
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	129.1	90.6	344.9	252.8
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	10.3	27.3	46.1	73.8
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	7.7	13.0	32.7	43.3
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 17.7	$ 15.5	$ 50.5	$ 43.8