Retirement Plans and Other Post-Retirement Benefits - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (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]
Actuarial (gain) loss	$ (8.3)	$ 1.4
Plan amendments	4.1	5.5
Recognized prior service cost	(2.8)	(2.7)	$ (3.1)
Recognized actuarial losses	(12.3)	(20.5)
Total recognized in other comprehensive (income) loss	(19.3)	(16.3)
Total recognized in net periodic benefit cost and other comprehensive (income) loss	(12.7)	(3.5)
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss	1.5	(3.4)
Recognized prior service cost	0.2	0.2	$ 0.3
Recognized actuarial losses	0.4	0.8
Total recognized in other comprehensive (income) loss	2.1	(2.4)
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ 4.7	$ (0.3)