Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0.9	$ 0.8
Interest cost	5.2	7.3
Expected return on assets	(8.1)	(9.1)
Settlement expense	0.4	0.1
Recognized actuarial (gain) loss	4.3	3.7
Net periodic benefit (credit) cost	2.7	2.8
Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0.1	0.1
Interest cost	0.4	0.6
Recognized actuarial (gain) loss	(1.7)	(1.9)
Amortization of prior service credit	(0.5)	(0.7)
Net periodic benefit (credit) cost	$ (1.7)	$ (1.9)