Postretirement Benefit Plans - Amount Recognized in Accumulated Other Comprehensive Income Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 80.1	$ 181.2
Prior service cost (benefit)	(26.7)	(31.8)
Total	53.4	149.4
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	40.9	143.4
Prior service cost (benefit)	0.4	0.4
Total	41.3	143.8
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	39.2	37.8
Prior service cost (benefit)	(27.1)	(32.2)
Total	$ 12.1	$ 5.6