Employee Benefit Plans - Components of Net Periodic Benefit (Income) Expense and Other Amounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net actuarial (gain) loss	$ (12.1)	$ 32.6	$ 5.5
Total pre-tax changes recognized in other comprehensive income (loss)	(20.4)	24.3	(3.0)
Pension Plans [Member]
Net periodic benefit (income) expense:
Interest cost	22.9	20.2	19.1
Expected return on plan assets	(46.1)	(44.3)	(37.9)
Recognized net actuarial loss	5.3	7.3	6.5
Recognized prior service credit	0.0	(0.3)	(0.8)
Settlements	2.9	1.0	2.6
Net periodic benefit (income) expense	(15.0)	(16.1)	(10.5)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net actuarial (gain) loss	(20.5)	26.0	(4.8)
Prior service credit	0.0	0.3	0.8
Total pre-tax changes recognized in other comprehensive income (loss)	(20.5)	26.3	(4.0)
Total recognized in net periodic benefit (income) expense and other comprehensive income (loss)	(35.5)	10.2	(14.5)
Other Postretirement Plans [Member]
Net periodic benefit (income) expense:
Interest cost	0.6	0.5
Expected return on plan assets	0.0	0.0	0.0
Recognized net actuarial loss	0.1	0.3	0.2
Recognized prior service credit	0.0	0.0	0.0
Settlements	0.0	0.0	0.0
Net periodic benefit (income) expense	1.0	1.1	1.0
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net actuarial (gain) loss	1.1	(2.0)	1.0
Prior service credit	0.0	0.0	0.0
Total pre-tax changes recognized in other comprehensive income (loss)	1.1	(2.0)	1.0
Total recognized in net periodic benefit (income) expense and other comprehensive income (loss)	2.1	(0.9)	2.0
Interest cost	$ 0.9	$ 0.8	$ 0.8