Income Taxes - Income Tax (Benefit) Provision Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current income tax provision (benefit) related to:
Total current income tax (benefit) provision	$ 35.1	$ 16.7	$ 7.5
Deferred income tax provision (benefit) related to:
Deferred income tax (benefit) provision, net	(26.5)	(12.6)	(17.9)
Income tax provision (benefit)	8.6	4.1	(10.4)
United States [Member]
Current income tax provision (benefit) related to:
Total current income tax (benefit) provision	36.5	13.4	(0.3)
Deferred income tax provision (benefit) related to:
Deferred income tax (benefit) provision, net	(19.3)	(13.8)	(0.3)
United Kingdom [Member]
Current income tax provision (benefit) related to:
Total current income tax (benefit) provision	(1.5)	3.2	7.6
Deferred income tax provision (benefit) related to:
Deferred income tax (benefit) provision, net	(7.2)	1.1	(17.6)
Other jurisdictions [Member]
Current income tax provision (benefit) related to:
Total current income tax (benefit) provision	0.1	0.1	0.2
Deferred income tax provision (benefit) related to:
Deferred income tax (benefit) provision, net	$ 0.0	$ 0.1	$ 0.0