Quarterly report pursuant to Section 13 or 15(d)

Revenue Recognition - Disaggregation of Revenue (Details)

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Revenue Recognition - Disaggregation of Revenue (Details) - USD ($)
$ in Thousands
 3 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disaggregation of Revenue [Line Items]    
Revenue $ 664,619 $ 623,148
United States of America    
Disaggregation of Revenue [Line Items]    
Revenue 511,200  
United Kingdom    
Disaggregation of Revenue [Line Items]    
Revenue 73,418  
Australia    
Disaggregation of Revenue [Line Items]    
Revenue 53,373  
Rest of world    
Disaggregation of Revenue [Line Items]    
Revenue 26,628  
Total U.S. state government agencies    
Disaggregation of Revenue [Line Items]    
Revenue 290,614  
New York State government agencies    
Disaggregation of Revenue [Line Items]    
Revenue 91,712  
Other U.S. state government agencies    
Disaggregation of Revenue [Line Items]    
Revenue 198,902  
United States Federal Government agencies    
Disaggregation of Revenue [Line Items]    
Revenue 198,278  
International government agencies    
Disaggregation of Revenue [Line Items]    
Revenue 142,781  
Other, including local municipalities and commercial customers    
Disaggregation of Revenue [Line Items]    
Revenue 32,946  
Performance-based    
Disaggregation of Revenue [Line Items]    
Revenue 312,887  
Cost-plus    
Disaggregation of Revenue [Line Items]    
Revenue 175,298  
Fixed price    
Disaggregation of Revenue [Line Items]    
Revenue 147,151  
Time and materials    
Disaggregation of Revenue [Line Items]    
Revenue 29,283  
Program administration | U.S. Health & Human Services    
Disaggregation of Revenue [Line Items]    
Revenue 218,973  
Program administration | U.S. Federal Services    
Disaggregation of Revenue [Line Items]    
Revenue 140,121  
Program administration | Outside the U.S.    
Disaggregation of Revenue [Line Items]    
Revenue 15,320  
Assessments and appeals | U.S. Health & Human Services    
Disaggregation of Revenue [Line Items]    
Revenue 37,221  
Assessments and appeals | U.S. Federal Services    
Disaggregation of Revenue [Line Items]    
Revenue 37,983  
Assessments and appeals | Outside the U.S.    
Disaggregation of Revenue [Line Items]    
Revenue 62,310  
Workforce and children services | U.S. Health & Human Services    
Disaggregation of Revenue [Line Items]    
Revenue 23,903  
Workforce and children services | Outside the U.S.    
Disaggregation of Revenue [Line Items]    
Revenue 73,278  
Other | U.S. Health & Human Services    
Disaggregation of Revenue [Line Items]    
Revenue 14,116  
Other | Outside the U.S.    
Disaggregation of Revenue [Line Items]    
Revenue 2,511  
Technology solutions | U.S. Federal Services    
Disaggregation of Revenue [Line Items]    
Revenue $ 38,883