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Debt and Credit Arrangements - Notes Interest Accretion Schedule (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Debt Instrument    
Interest accretion of convertible notes discount $ 1.8 $ 2.5
Financing costs amortization 0.4 0.3
Convertible Debt | Convertible Notes, due 2024    
Debt Instrument    
Interest accretion of convertible notes discount 1.8 1.7
Debt instrument, interest expense 0.6 0.6
Interest expense 2.4 2.3
Financing costs amortization $ 0.2 0.2
Convertible Debt | Convertible Notes, Due 2018    
Debt Instrument    
Interest accretion of convertible notes discount   0.8
Debt instrument, interest expense   0.6
Interest expense   $ 1.4