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Debt - Schedule of Interest Expense (Details) - Senior Notes - 5.50% convertible senior notes due 2021 - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Debt Instrument [Line Items]        
Contractual interest coupon $ 2.9 $ 2.9 $ 8.8 $ 8.8
Amortization of debt discount 1.7 1.6 4.9 4.5
Amortization of debt issuance costs 0.3 0.3 0.9 0.9
Total $ 4.9 $ 4.8 $ 14.6 $ 14.2