Debt and Credit Arrangements - Notes Interest Accretion Schedule (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Nov. 06, 2017
Debt Instrument
Interest accretion of convertible notes discount	$ 0.0	$ 8.0	$ 7.6
Total interest expense	11.7	18.1	17.7
Financing costs amortization	8.3	4.3	3.0
Convertible Debt | Convertible Notes, due 2024
Debt Instrument
Interest accretion of convertible notes discount	0.0	8.0	7.6
Interest expense excluding amortization	2.6	2.6	2.6
Total interest expense	2.6	10.6	10.2
Financing costs amortization	$ 0.9	$ 0.7	$ 0.7
Debt instrument stated interest rate (percent)	1.00%			1.00%