Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 500,085	$ 423,110	$ 983,367	$ 836,578
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	208,417	169,923	409,368	334,890
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	225,396	185,902	444,383	369,915
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	66,272	67,285	129,616	131,773
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	271,817	225,298	539,578	444,341
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	81,206	63,704	156,236	129,768
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	26,322	28,616	46,995	55,994
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	379,345	317,618	742,809	630,103
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 120,740	$ 105,492	$ 240,558	$ 206,475
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	42.00%	40.00%	42.00%	40.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	45.00%	44.00%	45.00%	44.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	13.00%	16.00%	13.00%	16.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	55.00%	53.00%	55.00%	53.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	16.00%	15.00%	16.00%	15.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	5.00%	7.00%	5.00%	7.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	76.00%	75.00%	76.00%	75.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	24.00%	25.00%	24.00%	25.00%
Energy, Environment, Infrastructure, and Disaster Recovery
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 202,723	$ 172,103	$ 389,254	$ 343,025
Energy, Environment, Infrastructure, and Disaster Recovery | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	41.00%	41.00%	40.00%	41.00%
Health and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 207,224	$ 159,165	$ 411,463	$ 314,964
Health and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	41.00%	37.00%	42.00%	38.00%
Security and Other Civilian & Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 90,138	$ 91,842	$ 182,650	$ 178,589
Security and Other Civilian & Commercial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	18.00%	22.00%	18.00%	21.00%