Retirement Plans, Postretirement and Postemployment Benefits - Schedule of Fair Values of Pension Plan Assets by Asset Class and Fair Value Hierarchy Level (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	$ 1,037.9	$ 868.0
Quoted Prices In Active Markets For Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	3.5	3.0
Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	1,034.4	865.0
Mid-sized to Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	302.3	262.5
Mid-sized to Large Cap | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	302.3	262.5
Small Cap, International and Emerging Growth Funds
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	328.4	290.3
Small Cap, International and Emerging Growth Funds | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	328.4	290.3
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	248.2	242.9
Debt Securities | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	248.2	242.9
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	80.4	42.9
Real Estate | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	80.4	42.9
Private Infrastructure
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	50.2
Private Infrastructure | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	50.2
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	24.9	26.4
Hedge Funds | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	24.9	26.4
Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	3.5	3.0
Cash Equivalents | Quoted Prices In Active Markets For Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	$ 3.5	$ 3.0