Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 512,029	$ 500,085	$ 1,006,465	$ 983,367
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	217,587	208,171	423,680	409,290
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	235,398	225,731	460,253	444,637
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	59,044	66,183	122,532	129,440
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	273,471	273,060	547,666	540,802
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	84,850	81,054	161,803	156,296
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	28,696	26,212	53,959	46,831
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	387,017	$ 380,326	763,428	743,929
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 125,012		$ 243,037	$ 239,438
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%	42.00%	42.00%	42.00%
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	12.00%	13.00%	12.00%	13.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	53.00%	55.00%	54.00%	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	17.00%	16.00%	16.00%	16.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	6.00%	5.00%	6.00%	5.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	76.00%	76.00%	76.00%	76.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	24.00%	24.00%	24.00%	24.00%
Energy, Environment, Infrastructure, and Disaster Recovery
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 232,655	$ 203,834	$ 457,260	$ 391,027
Energy, Environment, Infrastructure, and Disaster Recovery | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	45.00%	41.00%	45.00%	40.00%
Health and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 194,929	$ 205,530	$ 385,053	$ 408,239
Health and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	38.00%	41.00%	39.00%	41.00%
Security and Other Civilian & Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 84,445	$ 90,721	$ 164,152	$ 184,101
Security and Other Civilian & Commercial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	17.00%	18.00%	16.00%	19.00%