Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 378,478	$ 358,238
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	160,009	168,070
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	147,296	133,204
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	71,173	56,964
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	175,913	155,604
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	57,152	60,893
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	37,061	22,801
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	270,126	239,298
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 108,352	$ 118,940
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%	47.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	39.00%	37.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	19.00%	16.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	43.00%
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%	17.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	10.00%	7.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	71.00%	67.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	29.00%	33.00%
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 164,492	$ 148,961
Energy, Environmental and Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	43.00%	42.00%
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 157,765	$ 151,352
Health, Education and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	42.00%	42.00%
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 31,198	$ 30,301
Safety and Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	8.00%	8.00%
Consumer and Financial Services
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 25,023	$ 27,624
Consumer and Financial Services | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	8.00%