Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 451.5	$ 446.9	$ 1,303.7	$ 1,279.3
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	289.6	276.5	845.4	803.7
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	101.9	117.2	295.6	333.3
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	24.3	23.3	60.1	64.7
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	21.3	18.2	62.3	39.6
Other Jurisdiction [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 14.4	$ 11.7	$ 40.3	$ 38.0