Income Taxes - Deferred Taxes in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Net	$ 156.8	$ 136.0
Deferred Tax Assets, Net	156.8	136.0
Other Noncurrent Liabilities [Member]
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Net	1.5	2.1
Deferred Tax Assets, Net	1.5	2.1
Other Noncurrent Assets [Member]
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Net	158.3	138.1
Deferred Tax Assets, Net	$ 158.3	$ 138.1