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, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 50.8		$ 39.6		$ 93.5		$ 72.7
Effective Tax Rate		9.40%		22.10%		11.50%		19.40%
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 25.3		$ 22.1		$ 51.7		$ 44.1
Effective Tax Rate		0.00%		0.00%		0.00%		0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 34.7		$ 26.5		$ 58.7		$ 49.8
Effective Tax Rate		23.30%		30.90%		24.80%		27.50%
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ (6.5)		$ (9.8)		$ (12.0)		$ (22.1)
Effective Tax Rate		52.80%		(5.70%)		32.30%		(1.70%)
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0.1		$ 0.0	[1]	$ 0.1		$ 0.0	[1]
Effective Tax Rate		36.60%		16.30%		36.20%		32.00%
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0.6		$ 0.8		$ (0.2)		$ 0.7
Effective Tax Rate		0.00%		0.00%		0.00%		0.00%
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ (2.2)		$ 0.0	[1]	$ (3.5)		$ 0.0	[1]
Effective Tax Rate		0.00%		0.00%		0.00%		0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0.0	[1]	$ (0.1)		$ 0.0	[1]	$ (0.1)
Effective Tax Rate		0.00%		0.00%		0.00%		0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0.5		$ 0.1		$ 1.1		$ 0.3
Effective Tax Rate		0.00%		0.00%		0.00%		0.00%
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ (1.7)		$ 0.0		$ (2.4)		$ 0.0
Effective Tax Rate		0.00%		0.00%		0.00%		0.00%
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	[1]	$ 0.0		$ 0.0		$ 0.0		$ 0.0
Effective Tax Rate		22.10%		22.80%		20.80%		21.10%

[1] Pre-tax income for the respective year was less than $0.1 million.