Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 417.7	$ 399.1	$ 832.4	$ 778.5
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	20.9	23.1	41.4	48.7
Brazil [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	10.5	12.3	23.5	25.1
Italy [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	2.8	0.0	2.8	0.0
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	15.7	1.3	21.4	2.3
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	102.0	133.3	216.1	252.1
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 265.8	$ 229.1	$ 527.2	$ 450.3