Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 516,998	$ 501,519	$ 1,523,463	$ 1,484,886
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	220,564	206,622	644,822	615,902
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	236,538	223,338	696,211	667,982
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	59,896	71,559	182,430	201,002
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	282,022	279,314	829,688	820,116
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	78,883	76,594	240,685	233,264
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	26,871	27,547	80,831	74,378
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	387,776	383,455	1,151,204	1,127,758
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 129,222	$ 118,064	$ 372,259	$ 357,128
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	43.00%	41.00%	42.00%	41.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	45.00%	46.00%	45.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	11.00%	14.00%	12.00%	14.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	55.00%	56.00%	55.00%	55.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	15.00%	15.00%	16.00%	16.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	5.00%	5.00%	5.00%	5.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	75.00%	76.00%	76.00%	76.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	25.00%	24.00%	24.00%	24.00%
Energy, Environment, Infrastructure, and Disaster Recovery
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 236,039	$ 204,740	$ 693,229	$ 595,769
Energy, Environment, Infrastructure, and Disaster Recovery | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	41.00%	46.00%	40.00%
Health and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 196,586	$ 209,760	$ 582,191	$ 617,997
Health and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	38.00%	42.00%	38.00%	42.00%
Security and Other Civilian & Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 84,373	$ 87,019	$ 248,043	$ 271,120
Security and Other Civilian & Commercial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	16.00%	17.00%	16.00%	18.00%