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LONG-TERM DEBT - 4.0 Percent Convertible Notes (Details) - Convertible Notes - 4.00% convertible notes due 2027
12 Months Ended
Sep. 30, 2019
d
trading_day
$ / shares
$ / note
shares
Sep. 30, 2018
USD ($)
Feb. 28, 2007
USD ($)
Debt Instrument [Line Items]      
Stated interest rate 4.00%    
Debt instrument, face amount | $     $ 200,000,000
Equivalent shares of common stock per 1000 principal amount of convertible notes (in shares) | shares 37.4111    
Initial principal amount per note (in dollars per note) | $ / note 1,000    
Conversion price (in dollars per share) | $ / shares $ 26.73    
Maximum number of common stock convertible notes are convertible into (in shares) | shares 1,000,000    
Redemption option price 100.00%    
Redemption price 100.00%    
Debt outstanding, gross | $   $ 24,000,000  
Triggering Event One      
Debt Instrument [Line Items]      
Threshold trading days | d 20    
Threshold consecutive trading days | trading_day 30    
Threshold percentage of stock price trigger 120.00%    
Triggering Event Two      
Debt Instrument [Line Items]      
Threshold trading days | d 5    
Threshold consecutive trading days | trading_day 5    
Threshold percentage of stock price trigger 97.00%