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, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	$ 868.0	$ 717.9
Quoted Prices In Active Markets For Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	3.0	2.9
Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	865.0	715.0
Mid-sized to Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	262.5	196.5
Mid-sized to Large Cap | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	262.5	196.5
Small Cap, International and Emerging Growth Funds
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	290.3	210.4
Small Cap, International and Emerging Growth Funds | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	290.3	210.4
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	242.9	228.2
Debt Securities | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	242.9	228.2
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	42.9	35.5
Real Estate | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	42.9	35.5
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	26.4	44.4
Hedge Funds | Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	26.4	44.4
Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	3.0	2.9
Cash Equivalents | Quoted Prices In Active Markets For Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	$ 3.0	$ 2.9