Postretirement Benefit Plans - Amount Recognized in Accumulated Other Comprehensive Income Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 185.4	$ 197.4
Prior service cost (benefit)	(37.9)	(42.3)
Total	147.5	155.1
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	148.7	141.1
Prior service cost (benefit)	1.1	2.0
Total	149.8	143.1
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	36.7	56.3
Prior service cost (benefit)	(39.0)	(44.3)
Total	$ (2.3)	$ 12.0