Pensions and Postretirement Benefits - Schedule of Net Periodic Benefit Cost (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 Disclosure [Line Items]
Contributions				$ 20.0
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.2	$ 1.3	$ 2.3	2.5
Interest cost	3.1	3.4	6.1	6.8
Expected return on plan assets	(4.0)	(5.3)	(8.0)	(10.7)
Amortization of prior service cost	0.1	0.1	0.1	0.1
Amortization of net loss	1.7	1.5	3.3	2.9
Net periodic benefit cost	2.1	1.0	3.8	1.6
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.1	0.1
Interest cost	0.1	0.1	0.1	0.1
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of prior service cost	0.0	0.0	0.0	0.0
Amortization of net loss	0.0	0.0	0.0	0.0
Net periodic benefit cost	$ 0.1	$ 0.1	$ 0.2	$ 0.2