Pension Benefits and Savings Plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 0.3		$ 0.2
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer towards savings plan	8.9	$ 7.9	$ 7.0
Qualified Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Underfunded amount	3.7	$ 3.5
Nonqualified Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Underfunded amount	$ 2.0