Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 431.7	$ 417.7	$ 852.2	$ 832.4	$ 832.4
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	283.2	265.8	555.8		527.2
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	96.6	102.0	193.7		216.1
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	19.2	20.9	35.8		41.4
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	20.2	15.7	41.0		21.4
Other Jurisdiction [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 12.5	$ 13.3	$ 25.9		$ 26.3