Retirement Plans - Amounts in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (loss) gain	$ (297.4)	$ (1,229.9)
Net transition obligation	0.0	(0.1)
Net prior service credit	0.0	0.0
Total	(297.4)	(1,230.0)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (loss) gain	19.2	24.1
Net transition obligation	0.0	0.0
Net prior service credit	32.9	31.5
Total	$ 52.1	$ 55.6