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Debt - Convertible Senior Notes Due 2025 (Details)
12 Months Ended
May 14, 2018
USD ($)
day
$ / shares
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Debt Instrument [Line Items]      
Principal amount   $ 316,250,000  
Carrying value   264,186,000 $ 251,229,000
Debt discount and issuance costs   52,064,000 65,893,000
Senior Notes | 2025 Convertible Notes      
Debt Instrument [Line Items]      
Stated interest rate 2.375%    
Principal amount $ 172,500,000 $ 172,500,000  
Semi-annual interest rate 2.375%    
Conversion rate per $1,000 principal amount (shares) 62.8931    
Conversion price, per share (usd per share) | $ / shares $ 15.90    
Share price (usd per share) | $ / shares $ 15.90    
Remaining unamortized issuance discount period   7 years  
Carrying value $ 121,300,000 $ 140,643,000 133,164,000
Unamortized discount 51,200,000    
Equity component of debt, gross 49,600,000    
Conversion feature accounted for as an equity-classified instrument 39,100,000    
Debt transaction costs $ 5,500,000    
Effective interest rate   9.00%  
Debt discount and issuance costs   $ 31,857,000 $ 39,336,000
Senior Notes | 2025 Convertible Notes | Measurement Input, Discount Rate | Valuation Technique, Discounted Cash Flow      
Debt Instrument [Line Items]      
Debt instrument, expected term input   0.08  
Senior Notes | 2025 Convertible Notes | Debt Instrument, Redemption, Period One      
Debt Instrument [Line Items]      
Threshold consecutive trading days | day 30    
Threshold percentage of stock price trigger 130.00%    
Repurchase price, percent of principal amount 100.00%    
Senior Notes | 2025 Convertible Notes | Debt Conversion, Condition One      
Debt Instrument [Line Items]      
Threshold trading days (whether or not consecutive) | day 20    
Threshold consecutive trading days | day 30    
Threshold percentage of stock price trigger 130.00%    
Senior Notes | 2025 Convertible Notes | Debt Conversion, Condition Two      
Debt Instrument [Line Items]      
Threshold consecutive trading days | day 5    
Threshold percentage of stock price trigger 98.00%    
Business day threshold | day 5