Pensions and Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year			$ 1.8	$ 0.0
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year	$ 1.8		1.8
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	1.0	$ 0.9	2.1	2.2
Interest cost	3.0	3.2	6.0	6.3
Expected return on plan assets	(4.5)	(4.0)	(9.0)	(8.4)
Amortization of net loss	1.8	1.7	3.6	3.4
Deconsolidation of GST	(0.1)	(0.4)	(0.3)	(0.5)
Net periodic benefit cost	1.2	1.4	2.4	3.0
Other Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	0.0	0.1	0.0	0.2
Interest cost	0.0	0.1	0.0	0.2
Net periodic benefit cost	$ 0.0	$ 0.2	$ 0.0	$ 0.4