Changes in Benefit Obligation and Plan Assets (Detail) - Postretirement Benefits Other Than Pensions - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	$ 18.5	$ 19.3	$ 21.0
Service cost	0.4	0.4	0.5
Interest cost	0.7	0.8	0.8
Change in discount rate	1.6	0.7	(1.2)
Experience	(0.9)	(0.1)	(0.9)
Other assumption changes		(2.9)	(0.9)
Plan amendments	0.1	0.2	0.0
Benefits paid	(0.2)	(0.3)	(0.2)
Other	0.9	0.4	0.2
Benefit obligation at end of year	21.1	18.5	19.3
Company contributions	0.2	0.3	0.2
Accrued postretirement benefit cost recognized in the balance sheet	$ 21.1	$ 18.5	$ 19.3