Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 394,060	$ 360,315	$ 1,165,063	$ 1,072,540
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	157,571	168,250	479,632	504,772
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	166,121	135,260	473,901	392,525
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	70,368	56,805	211,530	175,243
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	195,107	175,126	553,232	501,525
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	58,516	50,819	173,451	169,001
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	32,249	20,194	107,175	60,861
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	285,872	246,139	833,858	731,387
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 108,188	$ 114,176	$ 331,205	$ 341,153
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%	47.00%	41.00%	47.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	42.00%	37.00%	41.00%	37.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	18.00%	16.00%	18.00%	16.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	49.00%	49.00%	48.00%	47.00%
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%	14.00%	15.00%	16.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	8.00%	5.00%	9.00%	5.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	72.00%	68.00%	72.00%	68.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	28.00%	32.00%	28.00%	32.00%
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 156,661	$ 147,188	$ 488,018	$ 445,592
Energy, Environmental and Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	40.00%	41.00%	42.00%	42.00%
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 182,334	$ 158,297	$ 508,255	$ 461,808
Health, Education and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	44.00%	44.00%	43.00%
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 28,304	$ 30,539	$ 89,120	$ 90,944
Safety and Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	8.00%	7.00%	8.00%
Consumer and Financial Services
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 26,761	$ 24,291	$ 79,670	$ 74,196
Consumer and Financial Services | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	7.00%	7.00%	7.00%