Pension and Other Post-Retirement Benefits - Components of Net Periodic Benefit Expense Related to Post-Retirement Plans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Net Periodic Benefit Cost [Asbtract]
Interest cost	$ 5.6	$ 4.9	$ 11.2	$ 9.6
Pension Plans [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Asbtract]
Service cost	4.0	4.6	8.3	9.4
Interest cost	5.2	4.4	10.3	8.8
Expected return on plan assets	(5.1)	(3.8)	(10.1)	(7.6)
Amortization of net actuarial loss from earlier periods	1.0	1.5	1.9	3.1
Amortization of net prior service costs from earlier periods	(0.1)	(0.1)	(0.2)	(0.2)
Net periodic expense	5.0	6.6	10.2	13.5
Other Retirement Plans [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Asbtract]
Service cost	0.7	0.8	1.4	1.5
Interest cost	0.3	0.2	0.6	0.5
Amortization of net prior service costs from earlier periods			(0.1)	(0.1)
Net periodic expense	$ 1.0	$ 1.0	$ 1.9	$ 1.9