Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 392,525	$ 353,987	$ 771,003	$ 712,225
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	162,368	168,371	322,146	336,521
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	160,169	124,141	307,696	257,265
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	69,988	61,475	141,161	118,439
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	182,188	170,786	358,100	326,389
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	58,058	57,388	114,961	118,280
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	37,801	17,858	74,925	40,667
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	278,047	246,032	547,986	485,336
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 114,478	$ 107,955	$ 223,017	$ 226,889
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	41.00%	48.00%	42.00%	47.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	41.00%	35.00%	40.00%	36.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	18.00%	17.00%	18.00%	17.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	48.00%	46.00%	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%	16.00%	15.00%	16.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	10.00%	5.00%	10.00%	6.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	71.00%	69.00%	71.00%	68.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	29.00%	31.00%	29.00%	32.00%
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 168,230	$ 150,234	$ 332,510	$ 299,050
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%	43.00%	42.00%
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 167,319	$ 152,036	$ 325,255	$ 303,388
Health, Education and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	43.00%	43.00%	42.00%	43.00%
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 29,618	$ 30,086	$ 60,816	$ 60,403
Safety and Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	9.00%	8.00%	8.00%
Consumer and Financial Services
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 27,358	$ 21,631	$ 52,422	$ 49,384
Consumer and Financial Services | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	6.00%	7.00%	7.00%