Annual report pursuant to Section 13 and 15(d)

Concentrations of credit risk and major customers - Schedule of Concentration of Risk, by Risk Factor (Details)

v3.10.0.1
Concentrations of credit risk and major customers - Schedule of Concentration of Risk, by Risk Factor (Details) - Total Revenue
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
State and local government agencies      
Concentration Risk [Line Items]      
Revenue from major customers (as a percent) 51.00% 49.00% 46.00%
Foreign government agencies      
Concentration Risk [Line Items]      
Revenue from major customers (as a percent) 27.00% 26.00% 26.00%
U.S. Federal Government agencies      
Concentration Risk [Line Items]      
Revenue from major customers (as a percent) 16.00% 19.00% 22.00%
Other sources      
Concentration Risk [Line Items]      
Revenue from major customers (as a percent) 6.00% 6.00% 6.00%
U.S. Federal Government      
Concentration Risk [Line Items]      
Revenue from major customers (as a percent) 16.00% 19.00% 22.00%
New York      
Concentration Risk [Line Items]      
Revenue from major customers (as a percent) 16.00% 15.00% 12.00%
United Kingdom      
Concentration Risk [Line Items]      
Revenue from major customers (as a percent) 12.00% 12.00% 16.00%
Australia      
Concentration Risk [Line Items]      
Revenue from major customers (as a percent) 10.00%