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, 2021
Dec. 31, 2020
Disaggregation of Revenue [Line Items]    
Revenue $ 1,150,876 $ 945,554
United States    
Disaggregation of Revenue [Line Items]    
Revenue 968,288 790,179
United Kingdom    
Disaggregation of Revenue [Line Items]    
Revenue 85,807 64,786
Australia    
Disaggregation of Revenue [Line Items]    
Revenue 52,814 55,932
Rest of world    
Disaggregation of Revenue [Line Items]    
Revenue 43,967 34,657
U.S. state government agencies    
Disaggregation of Revenue [Line Items]    
Revenue 371,547 388,114
United States Federal Government agencies    
Disaggregation of Revenue [Line Items]    
Revenue 564,094 385,573
International government agencies    
Disaggregation of Revenue [Line Items]    
Revenue 171,375 147,342
Other, including local municipalities and commercial customers    
Disaggregation of Revenue [Line Items]    
Revenue 43,860 24,525
Performance-based    
Disaggregation of Revenue [Line Items]    
Revenue 490,956 293,960
Cost-plus    
Disaggregation of Revenue [Line Items]    
Revenue 340,081 384,483
Fixed price    
Disaggregation of Revenue [Line Items]    
Revenue 151,505 120,777
Time and materials    
Disaggregation of Revenue [Line Items]    
Revenue $ 168,334 $ 146,334