Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 353,987	$ 366,717	$ 712,225	$ 707,971
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	123,970	146,967	257,122	281,416
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	168,489	167,009	336,639	322,191
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	61,528	52,741	118,464	104,364
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	170,748	141,253	326,318	273,465
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	57,982	73,101	119,288	138,928
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	18,098	31,617	41,190	58,751
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	246,828	245,971	486,796	471,144
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 107,159	$ 120,746	$ 225,429	$ 236,827
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 | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	35.00%	40.00%	36.00%	40.00%
Customer Concentration Risk | Revenue from Contract with Customer | Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	48.00%	46.00%	47.00%	45.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	17.00%	14.00%	17.00%	15.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	48.00%	38.00%	46.00%	39.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	17.00%	20.00%	17.00%	20.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	5.00%	9.00%	6.00%	8.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	70.00%	67.00%	69.00%	67.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	30.00%	33.00%	31.00%	33.00%
Energy Environmental And Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 151,481	$ 166,520	$ 303,839	$ 320,035
Energy Environmental And Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	43.00%	45.00%	43.00%	45.00%
Health Education And Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 148,528	$ 134,346	$ 291,114	$ 256,255
Health Education And Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	42.00%	37.00%	41.00%	36.00%
Safety And Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 31,440	$ 30,030	$ 63,025	$ 59,581
Safety And Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	9.00%	8.00%	9.00%	9.00%
Consumer And Financial Services
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 22,538	$ 35,821	$ 54,247	$ 72,100
Consumer And Financial Services | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	6.00%	10.00%	7.00%	10.00%