Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 425.8	$ 451.5	$ 431.7	$ 420.5	$ 452.4	$ 446.9	$ 417.7	$ 414.7	$ 1,729.5	$ 1,731.7	$ 1,572.3
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									1,115.6	1,073.1	930.8
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									391.5	455.8	489.3
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									80.7	85.4	92.1
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									85.0	61.0	11.1
Other Jurisdiction [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									$ 56.7	$ 56.4	$ 49.0