Long-term Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Convertible Notes, Due 2019
Debt Instrument [Line Items]
Contractual interest expense	$ 1.1	$ 1.1
Amortization of debt discount and deferred financing costs	2.6	2.5
Total interest expense	3.7	3.6
2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1.0	1.0
Amortization of debt discount and deferred financing costs	1.4	1.3
Total interest expense	2.4	2.3
2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1.0	0.0
Amortization of debt discount and deferred financing costs	1.6	0.0
Total interest expense	$ 2.6	$ 0.0