Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 45.3	$ (65.9)	$ 127.4	$ 27.6
Effective Tax Rate	10.20%	7.00%	15.80%	22.30%
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 6.4	$ (43.0)	$ (1.8)	$ 8.7
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 34.8	$ (1.3)	$ 114.2	$ 57.4
Effective Tax Rate	13.40%	52.90%	15.60%	24.10%
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 3.5	$ (24.6)	$ 16.3	$ (36.6)
Effective Tax Rate	(5.00%)	16.40%	12.30%	21.60%
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.0	$ 0.1	$ 0.0	$ 0.2
Effective Tax Rate	30.80%	36.10%	33.30%	36.20%
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.4	$ 0.0	$ (1.0)	$ (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	$ 3.5	$ 0.5	$ 0.0
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (0.1)	$ (0.1)	$ (0.2)	$ (0.1)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Italy [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (1.8)	$ 0.0	$ (2.9)	$ 0.0
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 1.8	$ 0.6	$ 2.2	$ 1.7
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.0	$ (1.3)	$ 0.0	$ (3.7)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.1	$ 0.2	$ 0.1	$ 0.2
Effective Tax Rate	21.20%	18.10%	21.20%	21.10%