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Debt - Schedule of Interest Expense (Details) - Senior Notes - Convertible Senior Notes 2021 - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Contractual interest coupon $ 11.8 $ 9.2
Amortization of debt discount 6.0 4.3
Amortization of debt issuance costs 1.2 1.0
Total $ 19.0 $ 14.5