Income Taxes - Roll-forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Income Tax Contingency [Line Items]
Opening balance at January 1	$ 16.4
Additions for tax positions of prior periods	0.0
Closing balance at 31 December	16.4
Current	0.0
Non-current	16.4	$ 16.4
Interest and Penalties [Member]
Income Tax Contingency [Line Items]
Opening balance at January 1	2.0
Additions for tax positions of prior periods	0.0
Closing balance at 31 December	2.0
Current	0.0
Non-current	2.0
Unrecognized Tax Benefits [Member]
Income Tax Contingency [Line Items]
Opening balance at January 1	14.4
Additions for tax positions of prior periods	0.0
Closing balance at 31 December	14.4
Current	0.0
Non-current	$ 14.4