Postretirement Benefit Plans - Net Loss and Prior Service Cost that will be Amortized from Accumulated Other Comprehensive Income Loss (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 10.2
Prior service cost (credit)	(4.4)
Total	5.8
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	5.9
Prior service cost (credit)	0.9
Total	6.8
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	4.3
Prior service cost (credit)	(5.3)
Total	$ (1.0)