Pension Plans and Other Postretirement Benefits - Future Benefits, Contribution (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Defined Benefit Plan Disclosure [Line Items]
2018	$ 9.4
2019	8.5
2020	11.2
2021	9.7
2022	11.9
2023 through 2027	73.3
Total	124.0
Estimated contributions to defined benefit pension plans	6.5
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
2018	9.3
2019	8.4
2020	11.0
2021	9.5
2022	11.6
2023 through 2027	71.1
Total	120.9
Other Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
2018	0.1
2019	0.1
2020	0.2
2021	0.2
2022	0.3
2023 through 2027	2.2
Total	$ 3.1