Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 467,777	$ 394,060	$ 1,304,355	$ 1,165,063
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	188,758	157,627	523,635	479,712
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	210,810	166,053	580,738	473,810
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	68,209	70,380	199,982	211,541
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	271,255	195,153	714,993	553,378
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	65,613	58,792	195,791	174,446
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	23,075	32,697	78,978	107,850
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	359,943	286,642	989,762	835,674
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 107,834	$ 107,418	$ 314,593	$ 329,389
Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	100.00%	100.00%	100.00%	100.00%
Customer Concentration Risk | Revenue from Contract with Customer | Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	40.00%	40.00%	40.00%	41.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	45.00%	42.00%	45.00%	41.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	15.00%	18.00%	15.00%	18.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	58.00%	50.00%	55.00%	48.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	14.00%	15.00%	15.00%	15.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	5.00%	8.00%	6.00%	9.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	77.00%	73.00%	76.00%	72.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	23.00%	27.00%	24.00%	28.00%
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 166,956	$ 156,384	$ 483,415	$ 486,945
Energy, Environmental and Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	36.00%	40.00%	37.00%	42.00%
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 249,034	$ 182,292	$ 663,739	$ 508,161
Health, Education and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	53.00%	46.00%	51.00%	43.00%
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 33,760	$ 28,311	$ 96,568	$ 89,130
Safety and Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	7.00%	7.00%	8.00%
Consumer and Financial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 18,027	$ 27,073	$ 60,633	$ 80,827
Consumer and Financial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	4.00%	7.00%	5.00%	7.00%