Retirement Plans, Postretirement and Postemployment Benefits - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for defined benefit pension plans	$ 878.7	$ 771.9
Combined pension plan and SERP contributions	58.9	162.3
Corporation's matching obligations	$ 17.6	16.5	$ 14.9
Defined contribution plan name	401(a)
Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Estimated contribution of pension plans				$ 60.2
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of pension asset equity securities in mid and large capitalization funds	45.00%
Pension
Defined Benefit Plan Disclosure [Line Items]
Prior service cost, recognized in net periodic benefit cost	$ 0.7
Prior service cost, recognized in net periodic benefit cost - deferred tax asset and liability	0.2
Actuarial gain (loss), recognized in net periodic benefit cost	13.5
Actuarial gain (loss), recognized in net periodic benefit cost - deferred tax asset and liability	3.3
Combined pension plan and SERP contributions	58.9	162.3
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost, recognized in net periodic benefit cost	0.8
Actuarial gain (loss), recognized in net periodic benefit cost	0.3
Combined pension plan and SERP contributions	2.0	$ 1.0
Prior service cost, recognized in net periodic benefit cost - deferred taxes	0.2
Actuarial gain (loss) , recognized in net periodic benefit cost - deferred taxes	$ 0.1
Postretirement Health Care Plans [Member] | Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Estimated contribution of pension plans				$ 2.0