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 9 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Disaggregation of Revenue [Line Items]        
Revenue $ 1,314,929 $ 1,188,677 $ 3,990,327 $ 3,644,775
U.S. federal government agencies        
Disaggregation of Revenue [Line Items]        
Revenue $ 666,895 $ 568,924 $ 2,015,780 $ 1,742,739
U.S. federal government agencies | Revenue Benchmark | Customer Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 50.70% 47.90% 50.50% 47.80%
U.S. state government agencies        
Disaggregation of Revenue [Line Items]        
Revenue $ 470,301 $ 445,855 $ 1,440,391 $ 1,329,766
U.S. state government agencies | Revenue Benchmark | Customer Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 35.80% 37.50% 36.10% 36.50%
International government agencies        
Disaggregation of Revenue [Line Items]        
Revenue $ 156,587 $ 148,742 $ 468,995 $ 491,861
International government agencies | Revenue Benchmark | Customer Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 11.90% 12.50% 11.80% 13.50%
Other, including local municipalities and commercial customers        
Disaggregation of Revenue [Line Items]        
Revenue $ 21,146 $ 25,156 $ 65,161 $ 80,409
Other, including local municipalities and commercial customers | Revenue Benchmark | Customer Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 1.60% 2.10% 1.60% 2.20%
Performance-based        
Disaggregation of Revenue [Line Items]        
Revenue $ 758,942 $ 617,800 $ 2,202,541 $ 1,761,764
Performance-based | Revenue Benchmark | Contract Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 57.70% 52.00% 55.20% 48.30%
Cost-plus        
Disaggregation of Revenue [Line Items]        
Revenue $ 281,823 $ 281,014 $ 958,336 $ 940,509
Cost-plus | Revenue Benchmark | Contract Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 21.40% 23.60% 24.00% 25.80%
Fixed price        
Disaggregation of Revenue [Line Items]        
Revenue $ 170,787 $ 171,809 $ 514,280 $ 527,556
Fixed price | Revenue Benchmark | Contract Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 13.00% 14.50% 12.90% 14.50%
Time and materials        
Disaggregation of Revenue [Line Items]        
Revenue $ 103,377 $ 118,054 $ 315,170 $ 414,946
Time and materials | Revenue Benchmark | Contract Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 7.90% 9.90% 7.90% 11.40%