Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 358,238	$ 341,254
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	133,128	134,448
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	168,213	155,182
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	56,896	51,624
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	155,375	132,092
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	61,330	65,950
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	23,092	27,135
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	239,797	225,177
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 118,441	$ 116,077
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	37.00%	40.00%
Customer Concentration Risk | Revenue from Contract with Customer | Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	47.00%	45.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	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	44.00%	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%	19.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	6.00%	8.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	67.00%	66.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	33.00%	34.00%
Energy Environmental And Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 151,362	$ 153,403
Energy Environmental And Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	42.00%	45.00%
Health Education And Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 143,423	$ 121,423
Health Education And Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	40.00%	35.00%
Safety And Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 31,585	$ 29,551
Safety And Security | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	9.00%	9.00%
Consumer And Financial Services
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 31,868	$ 36,877
Consumer And Financial Services | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	9.00%	11.00%