Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 435.0	$ 420.5
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	299.7	272.6
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	86.7	97.1
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	21.2	16.6
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	11.8	20.8
Other Jurisdiction [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 15.6	$ 13.4