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Share Based Compensation - Valuation Assumptions for Options Granted (Details) - $ / shares
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility, minimum 23.50% 25.30% 27.50%
Expected volatility, maximum 34.30% 28.70% 29.50%
Expected term (in years)     6 years 1 month 6 days
Risk free rate, minimum 2.50% 1.89% 1.01%
Risk free rate, maximum 2.80% 1.93% 1.42%
Grant date fair value per share of options granted (in dollars per share)    
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 6 years 6 years  
Grant date fair value per share of options granted (in dollars per share) $ 5.58 $ 2.13 $ 36.44
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 6 years 1 month 6 days 6 years 1 month 6 days  
Grant date fair value per share of options granted (in dollars per share) $ 7.96 $ 2.41 $ 61.77