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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, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Debt Instrument [Line Items]        
Interest rate 5.50%   5.50%  
Contractual interest coupon $ 2.9 $ 2.9 $ 8.8 $ 6.3
Amortization of debt discount 1.6 1.4 4.5 2.9
Amortization of debt issuance costs 0.3 0.3 0.9 0.7
Total $ 4.8 $ 4.6 $ 14.2 $ 9.9