Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement charge			$ 0.1	$ 0.4
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0.7	$ 0.9	1.4	1.8
Interest cost	2.4	2.9	4.9	5.9
Expected return on assets	(3.3)	(5.9)	(6.7)	(11.8)
Settlement charge		0.2	0.1	0.4
Recognized actuarial (gain) loss	2.0	3.9	4.0	7.8
Net periodic benefit cost (credit)	1.8	2.0	3.7	4.1
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0.1	0.1	0.2	0.2
Interest cost	0.3	0.3	0.6	0.6
Recognized actuarial (gain) loss	(1.4)	(1.5)	(2.9)	(3.0)
Amortization of prior service credit	(0.1)	(0.1)	(0.1)	(0.2)
Net periodic benefit cost (credit)	$ (1.1)	$ (1.2)	$ (2.2)	$ (2.4)