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 6 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2024
Mar. 31, 2023
Disaggregation of Revenue [Line Items]        
Revenue $ 1,348,357 $ 1,206,852 $ 2,675,398 $ 2,456,098
U.S. federal government agencies        
Disaggregation of Revenue [Line Items]        
Revenue $ 685,939 $ 569,897 $ 1,348,885 $ 1,173,815
U.S. federal government agencies | Revenue Benchmark | Customer Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 50.90% 47.20% 50.40% 47.80%
U.S. state government agencies        
Disaggregation of Revenue [Line Items]        
Revenue $ 483,147 $ 446,549 $ 970,090 $ 883,911
U.S. state government agencies | Revenue Benchmark | Customer Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 35.80% 37.00% 36.30% 36.00%
International government agencies        
Disaggregation of Revenue [Line Items]        
Revenue $ 156,796 $ 161,359 $ 312,408 $ 343,119
International government agencies | Revenue Benchmark | Customer Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 11.60% 13.40% 11.70% 14.00%
Other, including local municipalities and commercial customers        
Disaggregation of Revenue [Line Items]        
Revenue $ 22,475 $ 29,047 $ 44,015 $ 55,253
Other, including local municipalities and commercial customers | Revenue Benchmark | Customer Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 1.70% 2.40% 1.60% 2.20%
Performance-based        
Disaggregation of Revenue [Line Items]        
Revenue $ 738,888 $ 574,747 $ 1,443,599 $ 1,143,964
Performance-based | Revenue Benchmark | Contract Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 54.80% 47.60% 54.00% 46.60%
Cost-plus        
Disaggregation of Revenue [Line Items]        
Revenue $ 334,498 $ 312,176 $ 676,513 $ 659,495
Cost-plus | Revenue Benchmark | Contract Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 24.80% 25.90% 25.30% 26.90%
Fixed price        
Disaggregation of Revenue [Line Items]        
Revenue $ 166,816 $ 180,674 $ 343,493 $ 355,747
Fixed price | Revenue Benchmark | Contract Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 12.40% 15.00% 12.80% 14.50%
Time and materials        
Disaggregation of Revenue [Line Items]        
Revenue $ 108,155 $ 139,255 $ 211,793 $ 296,892
Time and materials | Revenue Benchmark | Contract Concentration Risk        
Disaggregation of Revenue [Line Items]        
Revenue in % 8.00% 11.50% 7.90% 12.10%