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Long-Term Debt - Interest Expense (Details) - 2.125% Cash convertible senior notes due 2019 - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Debt Instrument [Line Items]      
Interest expense $ 15.5 $ 14.1 $ 3.5
Contractual interest expense 4.9 4.9 1.4
Debt discount amortization $ 10.6 $ 9.2 $ 2.1
Effective interest rate 8.00% 8.00% 8.00%
Remaining unamortized issuance discount period 3 years