Income Taxes - Roll-Forward of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Opening balance at January 1	$ 2.2	$ 2.2
Additions for tax positions of prior periods	0.4	0.5
Reductions due to lapsed statute of limitations	0.0	(0.5)
Closing balance	$ 2.6	$ 2.2