Pensions and Post-Retirement Benefit Plans - Components of Net Periodic Benefit Cost for Pension Plans and Other Benefit Plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.5	$ 1.9	$ 2.0
Interest cost	7.8	7.5	9.0
Expected return on plan assets	(7.9)	(8.5)	(9.6)
Amortization of net loss (gain)	1.6	1.4	1.9
Settlements and curtailments	(0.0)	(0.0)	10.0
Net periodic benefit cost	3.0	2.3	13.3
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.1
Interest cost	0.4	0.4	0.4
Expected return on plan assets	(0.0)	(0.0)	(0.0)
Amortization of net loss (gain)	(0.2)	(0.0)	(0.2)
Settlements and curtailments	(0.0)	(0.0)	(0.0)
Net periodic benefit cost	$ 0.3	$ 0.5	$ 0.3