Postretirement Benefit Plans - Amount Recognized in Accumulated Other Comprehensive Income Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 58.4	$ 80.1
Prior service cost (benefit)	(21.7)	(26.7)
Total	36.7	53.4
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	30.0	40.9
Prior service cost (benefit)	0.3	0.4
Total	30.3	41.3
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	28.4	39.2
Prior service cost (benefit)	(22.0)	(27.1)
Total	$ 6.4	$ 12.1