Income Taxes - Activities Related to Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Balance at beginning of period		$ 18.8	$ 19.2	$ 15.6
Increase in balance related to tax positions taken during current year		1.0	1.4	4.6
Decrease in balance as a result of a lapse of the applicable statues of limitations	$ (2.4)	(0.1)	(2.2)	(1.0)
Decrease in balance due to settlement with tax authorities		(1.6)	0.0	0.0
Increase in balance related to tax positions taken during prior years		0.2	1.8	0.0
Decrease in balance related to tax positions taken during prior years		(0.3)	(1.4)	0.0
Balance at end of period	$ 18.8	$ 18.0	$ 18.8	$ 19.2