Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 445.5	$ 451.5	$ 1,313.9	$ 1,303.7
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	299.1	289.6	900.2	845.4
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	90.6	101.9	252.8	295.6
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	27.3	24.3	73.8	60.1
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	13.0	21.3	43.3	62.3
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 15.5	$ 14.4	$ 43.8	$ 40.3