Income Taxes - Activities Related to Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance at beginning of period	$ 17.0	$ 18.0	$ 18.8
Increase in balance related to tax positions taken during current year	0.3	0.2	1.0
Decrease in balance as a result of a lapse of the applicable statutes of limitations	0.0	(0.1)	(0.1)
Decrease in balance due to settlement with tax authorities	0.0	0.0	(1.6)
Increase in balance related to tax positions taken during prior years	0.3	0.0	0.2
Decrease in balance related to tax positions taken during prior years	0.0	(1.1)	(0.3)
Balance at end of period	$ 17.6	$ 17.0	$ 18.0