Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (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
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 29.6	$ 57.1	$ 129.7	$ 82.1	$ 82.1
Effective Tax Rate	2.90%	26.90%	7.50%	18.90%
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 15.0	$ (33.6)	$ 47.0	$ (8.4)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 22.3	$ 85.7	$ 91.8	$ 79.5
Effective Tax Rate	14.80%	17.10%	14.50%	16.60%
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (8.9)	$ 6.6	$ (12.4)	$ 12.7
Effective Tax Rate	26.70%	10.10%	28.10%	17.10%
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (0.3)	$ (0.1)	$ (0.3)	$ 0.0
Effective Tax Rate	29.50%	36.40%	29.70%	35.80%
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 2.0	$ 0.1	$ 3.8	$ (1.3)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.0	$ 0.2	$ 0.2	$ 0.4
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.0	$ (0.1)	$ (0.1)	$ (0.1)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Italy [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.2	$ (1.1)	$ (0.5)	$ (1.1)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (0.7)	$ (0.6)	$ 0.2	$ 0.4
Effective Tax Rate	(0.10%)	(0.20%)	0.50%	0.20%
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.0	$ 0.0	$ 0.0	$ 0.0
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	0.00%	20.80%	0.00%	20.80%