Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 423,110	$ 392,525	$ 836,578	$ 771,003
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	169,980	162,249	334,821	322,085
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	185,823	160,288	369,962	307,757
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	67,307	69,988	131,795	141,161
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	225,229	182,241	444,221	358,225
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	64,056	58,487	130,165	115,637
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	27,144	37,148	53,863	73,894
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	316,429	277,876	628,249	547,756
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 106,681	$ 114,649	$ 208,329	$ 223,247
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%	41.00%	40.00%	42.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	44.00%	41.00%	44.00%	40.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	16.00%	18.00%	16.00%	18.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	53.00%	46.00%	53.00%	46.00%
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%	15.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	10.00%	6.00%	10.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	75.00%	71.00%	75.00%	71.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	25.00%	29.00%	25.00%	29.00%
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 160,401	$ 167,309	$ 316,444	$ 330,533
Energy, Environmental and Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	38.00%	43.00%	38.00%	43.00%
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 208,953	$ 167,640	$ 414,750	$ 325,906
Health, Education and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	49.00%	43.00%	50.00%	42.00%
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 33,409	$ 29,618	$ 62,809	$ 60,816
Safety and Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	8.00%	7.00%	7.00%	8.00%
Consumer and Financial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 20,347	$ 27,958	$ 42,575	$ 53,748
Consumer and Financial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	5.00%	7.00%	5.00%	7.00%