Postretirement Benefit Plans - Amount Recognized in Accumulated Other Comprehensive Income Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 181.2	$ 185.4
Prior service cost (benefit)	(31.8)	(37.9)
Total	149.4	147.5
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	143.4	148.7
Prior service cost (benefit)	0.4	1.1
Total	143.8	149.8
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	37.8	36.7
Prior service cost (benefit)	(32.2)	(39.0)
Total	$ 5.6	$ (2.3)