Income Taxes - Classification of Net Deferred Income Tax Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Long-term deferred income tax assets	$ 528.8	$ 646.8
Long-term deferred income tax liabilities	(83.3)	(91.4)
Net deferred income tax asset	$ 445.5	$ 555.4