Changes in Accumulated Other Comprehensive Income (Loss) - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Actuarial gains (losses), net of tax	$ 1.3	$ (3.0)	$ (0.6)
Actuarial (losses) gains, tax expense (benefit)	0.1	(1.6)	(0.4)
Income tax expense (benefit) related to revaluation of deferred tax liability	4.5
Amortization of actuarial losses, net of tax	0.7	0.6	0.5
Amortization of actuarial losses, tax expense (benefit)	$ (0.4)	$ (0.4)	$ (0.3)