Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 360,315	$ 373,918	$ 1,072,540	$ 1,081,889
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	134,980	135,720	391,888	417,016
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	168,458	183,872	505,185	506,183
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	56,877	54,326	175,467	158,690
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	175,093	148,339	501,496	421,696
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	51,589	71,505	170,617	209,993
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	20,299	28,003	61,421	86,839
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	246,981	247,847	733,534	718,528
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 113,334	$ 126,071	$ 339,006	$ 363,361
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	37.00%	36.00%	37.00%	38.00%
Customer Concentration Risk | Revenue from Contract with Customer | Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	47.00%	49.00%	47.00%	47.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	16.00%	15.00%	16.00%	15.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	49.00%	40.00%	47.00%	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	14.00%	19.00%	16.00%	19.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	6.00%	7.00%	6.00%	8.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	69.00%	66.00%	69.00%	66.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	31.00%	34.00%	31.00%	34.00%
Energy Environmental And Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 148,286	$ 170,640	$ 451,404	$ 485,925
Energy Environmental And Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	41.00%	46.00%	42.00%	45.00%
Health Education And Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 159,508	$ 143,433	$ 457,553	$ 408,712
Health Education And Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	44.00%	38.00%	43.00%	38.00%
Safety And Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 29,281	$ 29,416	$ 89,978	$ 88,249
Safety And Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	8.00%	8.00%	8.00%	8.00%
Consumer And Financial Services
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 23,240	$ 30,429	$ 73,605	$ 99,003
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%	7.00%	9.00%