Postretirement Benefit Plans - Net Loss and Prior Service Cost that will be Amortized from Accumulated Other Comprehensive Income Loss (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 7.5
Prior service cost (credit)	(4.6)
Total	2.9
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	5.0
Prior service cost (credit)	0.7
Total	5.7
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	2.5
Prior service cost (credit)	(5.3)
Total	$ (2.8)