Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 483,282	$ 413,468
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	200,990	164,968
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	219,016	184,012
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	63,276	64,488
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	267,742	219,044
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	74,933	66,117
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	20,669	27,377
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	363,344	312,538
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 119,938	$ 100,930
Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	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%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	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%
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%
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%	16.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	4.00%	7.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	75.00%	76.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	25.00%	24.00%
Energy, Environment, Infrastructure, and Disaster Recovery
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 187,396	$ 169,028
Energy, Environment, Infrastructure, and Disaster Recovery | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	39.00%	41.00%
Health and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 203,698	$ 156,057
Health and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	42.00%	38.00%
Security and Other Civilian & Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 92,188	$ 88,383
Security and Other Civilian & Commercial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	19.00%	21.00%