Pension Benefits and Savings Plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 0.1	$ 0.1	$ 0.3
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer towards savings plan	9.3	9.2	$ 8.9
Qualified Plan | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Underfunded amount	2.6	4.2
Nonqualified Plan | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Underfunded amount	$ 1.8	$ 2.0