Employee Benefit Plans - Components of Net Periodic Benefit (Income) Expense and Other Amounts (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 4.9	$ 5.6
Expected return on plan assets	(12.4)	(11.4)
Recognized net actuarial loss	2.0	1.3
Settlements	0.8	0.6
Net periodic benefit (income) expense	(4.7)	(3.9)
Net actuarial loss	(2.0)	(1.3)
Prior service credit	0.0	0.0
Total pre-tax changes recognized in other comprehensive income (loss)	(2.0)	(1.3)
Total recognized in net periodic benefit (income) expense and other comprehensive income (loss)	(6.7)	(5.2)
Other Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.3	0.3
Expected return on plan assets	0.0	0.0
Recognized net actuarial loss	0.0	0.0
Settlements	0.0	0.0
Net periodic benefit (income) expense	0.3	0.3
Net actuarial loss	0.0	0.0
Prior service credit	0.0	0.0
Total pre-tax changes recognized in other comprehensive income (loss)	0.0	0.0
Total recognized in net periodic benefit (income) expense and other comprehensive income (loss)	$ 0.3	$ 0.3