Debt and Credit Arrangements - Notes Interest Accretion Schedule (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 06, 2017
Debt Instrument
Interest accretion of convertible notes discount	$ 8.0	$ 7.6	$ 9.1
Total interest expense	18.1	17.7	15.9
Financing costs amortization	4.3	3.0	$ 1.3
Convertible Debt \| Convertible Notes, due 2024
Debt Instrument
Interest accretion of convertible notes discount	8.0	7.6
Interest expense excluding amortization	2.6	2.6
Total interest expense	10.6	10.2
Financing costs amortization	$ 0.7	$ 0.7
Debt instrument stated interest rate (percent)	1.00%			1.00%

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of amortization expense attributable to debt issuance costs.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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