Postretirement Benefit Plans - Net Loss and Prior Service Cost that will be Amortized from Accumulated Other Comprehensive Income Loss (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 11.0
Prior service cost (credit)	(4.9)
Total	6.1
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	6.8
Prior service cost (credit)	0.9
Total	7.7
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	4.2
Prior service cost (credit)	(5.8)
Total	$ (1.6)