Schedule of Changes in Benefit Obligation (Detail) - Postretirement Benefits Other Than Pensions - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	$ 18.4	$ 15.5
Service cost	0.2	0.2	$ 0.3
Interest cost	0.6	0.6	0.6
Actuarial loss (gain)	2.1	2.2
Benefits paid	(0.4)	(0.3)
Other	0.3	0.2
Benefit obligation at end of year	$ 21.2	$ 18.4	$ 15.5