Income Taxes - Net Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred income taxes	$ 7.8	$ 15.5
Other noncurrent liabilities	1.3	1.0
Net unrecognized tax benefits	9.1	16.5
Gross unrecognized tax benefits	$ 9.1	$ 16.5	$ 14.0	$ 12.7