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, 2019	Dec. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss	$ (5.2)	$ 32.9
Plan amendments	(0.2)	0.1
Recognized prior service costs	(0.2)
Recognized actuarial losses	(78.1)	(7.1)
Total recognized in other comprehensive (income) loss	(83.7)	25.9
Total recognized in net periodic benefit cost and other comprehensive (income) loss	(5.0)	27.5
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss	(0.3)	(3.1)
Recognized actuarial losses	0.9	0.3
Total recognized in other comprehensive (income) loss	$ 0.6	(2.8)
Total recognized in net periodic benefit cost and other comprehensive (income) loss		$ (2.5)