Net Periodic Benefit Expense Amortized from Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Settlement charge	$ 0.0	$ 31.3	$ 0.0
Interest cost	3.1	5.4	6.3
Expected return on plan assets	(0.8)	(2.7)	(5.0)
Amortization of net actuarial loss	2.0	2.6	4.3
Amortization of prior service cost (credit)	0.1	0.1	0.1
Net periodic benefit expense	4.4	36.7	5.7
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Settlement charge	0.0	0.0	0.0
Interest cost	0.0	0.0	0.0
Expected return on plan assets	0.0	0.0	0.0
Amortization of net actuarial loss	0.1	0.1	0.1
Amortization of prior service cost (credit)	0.0	0.0	0.0
Net periodic benefit expense	$ 0.1	$ 0.1	$ 0.1