Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (103.3)	$ (26.2)	$ 29.6	$ 100.1	$ (59.7)	$ 45.3	$ 57.1	$ 25.0	$ 0.2	$ 67.7	$ 39.9
Effective Tax Rate									0.00%	6.10%	(26.10%)
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ (34.8)	$ 26.0	$ 30.2
Effective Tax Rate									0.00%	0.00%	0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 85.2	$ 13.9	$ 67.5
Effective Tax Rate									19.70%	(5.50%)	(0.90%)
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ (46.1)	$ 25.0	$ (53.8)
Effective Tax Rate									17.90%	18.80%	18.70%
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0.0	$ 0.0	$ 0.1
Effective Tax Rate									15.80%	0.00%	75.00%
Belgium [Member] | Maximum [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0.1
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 5.2	$ (0.5)	$ 0.8
Effective Tax Rate									0.00%	0.00%	0.00%
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0.4	$ 0.8	$ 0.2
Effective Tax Rate									0.00%	0.00%	0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ (0.1)	$ (0.2)	$ (0.2)
Effective Tax Rate									0.00%	0.00%	0.00%
Italy [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ (7.4)	$ 0.9	$ 0.0
Effective Tax Rate									0.00%	0.00%	0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ (2.0)	$ 1.7	$ 0.3
Effective Tax Rate									0.00%	0.00%	0.00%
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0.0	$ 0.0	$ (5.2)
Effective Tax Rate									0.00%	0.00%	0.00%
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ (0.2)	$ 0.1	$ 0.0
Effective Tax Rate									(0.20%)	18.40%	21.10%