Income Taxes - Schedule of Unrecognized Tax Benefits Excluding Interest Correlative Effects (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Unrecognized tax benefits at beginning of year	$ 25.5	$ 24.1	$ 22.4
Gross increases – tax positions in prior years	0.2	0.4	0.9
Gross increases – tax positions in current year	0.1	1.8	1.8
Gross decreases – tax positions in current year	(0.2)	(0.8)	(1.0)
Lapse of statute of limitations	(17.4)
Unrecognized tax benefits at end of year	8.2	25.5	24.1
Amount that, if recognized, would favorably impact the effective tax rate	$ 6.4	$ 15.5	$ 12.8