Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 413,468	$ 378,478
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	165,084	159,836
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	183,896	147,469
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	64,488	71,173
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	220,343	175,985
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	64,833	56,878
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	26,720	36,746
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	311,896	269,609
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 101,572	$ 108,869
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	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%	39.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	16.00%	19.00%
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%
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%	15.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	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%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	25.00%	29.00%
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 156,644	$ 163,582
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%
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 205,532	$ 158,268
Health, Education and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	50.00%	42.00%
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 29,400	$ 31,198
Safety and Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	8.00%
Consumer and Financial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 21,892	$ 25,430
Consumer and Financial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	5.00%	7.00%