Annual report pursuant to Section 13 and 15(d)

Concentrations of credit risk and major customers - Narrative (Details)

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Concentrations of credit risk and major customers - Narrative (Details) - Total Revenue
12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Foreign customers      
Revenue Recognition      
Revenue from major customers (as a percent) 28.00% 26.00% 23.00%
State government agencies      
Revenue Recognition      
Revenue from major customers (as a percent) 46.00%    
Foreign government agencies      
Revenue Recognition      
Revenue from major customers (as a percent) 26.00%    
United States Based Federal Government Agencies      
Revenue Recognition      
Revenue from major customers (as a percent) 22.00%    
Other sources including local municipalities and commercial customers      
Revenue Recognition      
Revenue from major customers (as a percent) 6.00%    
U.S. Federal      
Revenue Recognition      
Revenue from major customers (as a percent) 22.00% 20.00% 17.00%
United Kingdom Government      
Revenue Recognition      
Revenue from major customers (as a percent) 16.00%    
State of New York      
Revenue Recognition      
Revenue from major customers (as a percent) 12.00% 10.00%  
Minimum | U.S. Federal      
Revenue Recognition      
Revenue from major customers (as a percent) 10.00%    
Minimum | United Kingdom Government      
Revenue Recognition      
Revenue from major customers (as a percent) 10.00%    
Minimum | State of New York      
Revenue Recognition      
Revenue from major customers (as a percent) 10.00%