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Debt - Convertible Senior Notes Due 2025 (Details)
12 Months Ended
May 14, 2018
USD ($)
day
Sep. 30, 2019
USD ($)
$ / shares
Sep. 30, 2018
USD ($)
Oct. 01, 2017
$ / shares
Debt Instrument [Line Items]        
Principal amount   $ 317,976,000    
Share price (usd per share) | $ / shares   $ 10.70   $ 9.50
Carrying value   $ 238,594,000 $ 416,883,000  
Debt discount and issuance costs   79,382,000 98,223,000  
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 $ 126,210,000 119,736,000  
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   $ 46,290,000 $ 52,764,000  
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%      
Debt Conversion, Condition One | Senior Notes | 2025 Convertible Notes        
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%      
Debt Conversion, Condition Two | Senior Notes | 2025 Convertible Notes        
Debt Instrument [Line Items]        
Threshold consecutive trading days | day 5      
Threshold percentage of stock price trigger 98.00%      
Business day threshold | day 5      
Measurement Input, Discount Rate | Valuation Technique, Discounted Cash Flow | Senior Notes | 2025 Convertible Notes        
Debt Instrument [Line Items]        
Debt instrument, expected term input   0.08