Postretirement Benefit Plans - Amount Recognized in Accumulated Other Comprehensive Income Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 197.4	$ 213.6
Prior service cost (benefit)	(42.3)	(45.4)
Total	155.1	168.2
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	141.1	154.0
Prior service cost (benefit)	2.0	5.1
Total	143.1	159.1
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	56.3	59.6
Prior service cost (benefit)	(44.3)	(50.5)
Total	$ 12.0	$ 9.1