Employee Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit costs and other current liabilities	$ (0.7)	$ (0.6)
Deferred credits and other liabilities	(80.0)	(69.3)
Net amount recognized at end of year	(80.7)	(69.9)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit costs and other current liabilities	(9.5)	(9.2)
Deferred credits and other liabilities	(138.8)	(142.3)
Net amount recognized at end of year	$ (148.3)	$ (151.5)