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Debt And Credit Arrangements - Notes Interest Accretion Schedule (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Debt Instrument    
Interest accretion of convertible notes discount $ 2.5 $ 3.3
Financing costs amortization 0.3 0.3
Convertible Debt | Convertible Notes, due 2024    
Debt Instrument    
Interest accretion of convertible notes discount 1.7  
Debt instrument, interest expense 0.6  
Interest expense 2.3  
Financing costs amortization 0.2  
Convertible Debt | Convertible Notes, Due 2018    
Debt Instrument    
Interest accretion of convertible notes discount 0.8 3.3
Debt instrument, interest expense 0.6 1.3
Interest expense 1.4 4.6
Financing costs amortization $ 0.0 $ 0.2