Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 373,918	$ 332,968	$ 1,081,889	$ 960,063
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	135,274	126,126	415,258	384,751
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	184,330	146,525	507,926	398,815
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	54,314	60,317	158,705	176,497
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	148,181	140,276	421,287	413,599
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	71,540	55,525	210,324	121,114
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	28,020	26,559	86,885	89,950
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	247,741	222,360	718,496	624,663
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 126,177	$ 110,608	$ 363,393	$ 335,400
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	36.00%	38.00%	38.00%	40.00%
Customer Concentration Risk | Revenue from Contract with Customer | Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	49.00%	44.00%	47.00%	42.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	15.00%	18.00%	15.00%	18.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	40.00%	42.00%	39.00%	43.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	19.00%	17.00%	19.00%	13.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	7.00%	8.00%	8.00%	9.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	66.00%	67.00%	66.00%	65.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	34.00%	33.00%	34.00%	35.00%
Energy Environmental And Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 172,217	$ 147,914	$ 492,509	$ 404,870
Energy Environmental And Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	44.00%	46.00%	42.00%
Health Education And Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 138,699	$ 126,424	$ 394,288	$ 381,063
Health Education And Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	37.00%	38.00%	36.00%	40.00%
Safety And Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 29,682	$ 29,478	$ 88,913	$ 82,444
Safety And Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	8.00%	9.00%	8.00%	8.00%
Consumer And Financial Services
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 33,320	$ 29,152	$ 106,179	$ 91,686
Consumer And Financial Services | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	9.00%	9.00%	10.00%	10.00%