Retirement Plans - Amounts Recognized in Other Comprehensive Loss (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Amortization of Net actuarial (gain) loss	$ 7.3
Amortization of Net prior service credit	0.0
Amounts arising during the period, Net actuarial gain	1.8
Amounts arising during the period, Settlements	1.6
Total	10.7
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Amortization of Net actuarial (gain) loss	(0.1)
Amortization of Net prior service credit	(2.8)
Amounts arising during the period, Net actuarial gain	14.8
Amounts arising during the period, Settlements	0.0
Total	$ 11.9