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Financing Arrangements - Schedule of Interest Expense (Details) - Convertible Senior Notes - Convertible Senior Notes, due 2021 (6.00% fixed rate) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Contractual interest expense $ 5.2 $ 5.2
Amortization of debt issuance costs 0.4 0.5
Amortization of debt discount 3.6 3.2
Total $ 9.2 $ 8.9