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Stockholders' Equity - Narrative (Details) - USD ($)
 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Class of Stock [Line Items]    
Stock repurchased (in shares) 3,137,178 3,867,563
Additional shares authorized to be repurchased (in shares) 1,902,412  
Warrants issued (in shares) 1,707,456  
Warrant exercise price (in dollars per share) $ 17.57  
Warrants repurchased during period (in shares) 478,399  
Value of warrants $ 7,631,576  
Warrants outstanding (in shares) 330,217 808,616
Weighted Average    
Class of Stock [Line Items]    
Weighted average cost of repurchased stock (in dollars per share) $ 31.36 $ 22.72