Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 57.1	$ 50.8	$ 82.1	$ 93.5
Effective Tax Rate	26.90%	9.40%	18.90%	11.50%
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (33.6)	$ 25.3	$ (8.4)	$ 51.7
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 85.7	$ 34.7	$ 79.5	$ 58.7
Effective Tax Rate	17.10%	23.30%	16.60%	24.80%
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 6.6	$ (6.5)	$ 12.7	$ (12.0)
Effective Tax Rate	10.10%	52.80%	17.10%	32.30%
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (0.1)	$ 0.1	$ 0.0	$ 0.1
Effective Tax Rate	36.40%	36.60%	0.00%	36.20%
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.1	$ 0.6	$ (1.3)	$ (0.2)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.2	$ (2.2)	$ 0.4	$ (3.5)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (0.1)	$ 0.0	$ (0.1)	$ 0.0
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Italy [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (1.1)	$ 0.0	$ (1.1)	$ 0.0
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (0.6)	$ 0.5	$ 0.4	$ 1.1
Effective Tax Rate	(0.20%)	0.00%	0.20%	0.00%
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.0	$ (1.7)	$ 0.0	$ (2.4)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Effective Tax Rate	20.80%	22.10%	20.80%	20.80%