Income Taxes - Gross Unrecognized Tax Benefit (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Gross unrecognized tax benefits at January	$ 14.5	$ 15.7
Increases in tax positions for current year	1.6	2.4
Settlements	(1.8)	(1.4)
Lapse in statute of limitations	(0.7)	(1.8)
Increases in tax positions for prior years	0.6	1.8
Decreases in tax positions for prior years	(3.3)	(2.2)
Gross unrecognized tax benefits at December	$ 10.9	$ 14.5