Retirement Plans - Components of Net Pension and Postretirement Benefit Expense (Income) and Total (Income) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.0	$ 1.0	$ 0.7
Interest cost	27.4	33.3	31.3
Expected return on plan assets	(40.8)	(46.3)	(50.3)
Amortization of prior service credit	0.1		0.0
Amortization of actuarial loss (gain)	10.0	6.1	7.9
Settlements and curtailments	1.4	(0.1)	1.9
Net periodic income	$ (0.9)	$ (6.0)	$ (8.5)
Discount rate	3.00%	4.00%	3.40%
Expected return on plan assets	4.60%	5.20%	5.50%
OPEB
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.0	$ 0.0	$ (3.7)
Interest cost	7.3	10.1	9.4
Expected return on plan assets	(12.6)	(13.2)	(13.9)
Amortization of prior service credit	(5.4)	(5.4)	(3.3)
Amortization of actuarial loss (gain)	(0.7)	(1.7)	(0.6)
Net periodic income	$ (11.4)	$ (10.2)	$ (12.1)
Discount rate	3.00%	4.20%	3.50%
Expected return on plan assets	6.30%	6.50%	6.80%