Postretirement Benefit Plans - Amount Recognized in Accumulated Other Comprehensive Income Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 213.6	$ 235.5
Prior service cost (benefit)	(45.4)	(51.8)
Total	168.2	183.7
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	154.0	168.9
Prior service cost (benefit)	5.1	5.6
Total	159.1	174.5
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	59.6	66.6
Prior service cost (benefit)	(50.5)	(57.4)
Total	$ 9.1	$ 9.2