Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 446.9	$ 389.3	$ 1,279.3	$ 1,167.8
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	276.5	242.5	803.7	692.8
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	117.2	109.1	333.3	361.2
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	23.3	23.3	64.7	72.0
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	18.2	2.7	39.6	5.0
Other Jurisdiction [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 11.7	$ 11.7	$ 38.0	$ 36.8