Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 341,254	$ 302,780
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	133,534	122,328
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	156,347	124,342
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	51,373	56,110
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	131,823	134,185
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	65,522	31,313
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	27,158	28,767
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	224,503	194,265
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 116,751	$ 108,515
Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	100.00%	100.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	39.00%	40.00%
Customer Concentration Risk | Revenue from Contract with Customer | Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	41.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	15.00%	19.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	39.00%	44.00%
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%	10.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	8.00%	10.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	66.00%	64.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	34.00%	36.00%
Energy Environmental And Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 153,651	$ 124,250
Energy Environmental And Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	45.00%	41.00%
Health Education And Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 121,337	$ 122,928
Health Education And Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	35.00%	41.00%
Safety And Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 29,052	$ 25,475
Safety And Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	9.00%	8.00%
Consumer And Financial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 37,214	$ 30,127
Consumer And Financial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	11.00%	10.00%