Debt and Credit Arrangements - Notes Interest Accretion Schedule (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Nov. 06, 2017
Debt Instrument
Interest accretion of convertible notes discount			$ 0.0	$ 3.9
Interest expense excluding amortization	$ 2.2	$ 7.6	4.2	14.8
Financing costs amortization	1.1	1.1	2.3	2.1
Convertible Debt | Convertible Notes, due 2024
Debt Instrument
Interest accretion of convertible notes discount	0.0	2.0	0.0	3.9
Interest expense excluding amortization	0.7	0.7	1.3	1.3
Total interest expense	0.7	2.7	1.3	5.2
Financing costs amortization	$ 0.3	$ 0.2	$ 0.5	$ 0.4
Debt instrument stated interest rate (percent)					1.00%