Retirement Plans and Other Post-Retirement Benefits - Schedule of Net Periodic Benefit Cost of Pension and Other Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 10.7	$ 10.5	$ 11.6
Interest cost	23.8	24.5	23.3
Expected return on plan assets	(43.0)	(45.4)	(46.0)
Amortization of prior service cost/(credit)	2.8	2.7	3.1
Amortization of actuarial loss	12.3	13.2	17.1
One-time settlement charge		7.3
Total net periodic benefit cost	6.6	12.8	9.1
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.2	1.1	1.4
Interest cost	2.0	2.0	2.0
Amortization of prior service cost/(credit)	(0.2)	(0.2)	(0.3)
Amortization of actuarial loss	(0.4)	(0.8)	0.2
Total net periodic benefit cost	$ 2.6	$ 2.1	$ 3.3