Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 366,717,000	$ 324,315,000	$ 707,971,000	$ 627,095,000
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	146,516,000	136,220,000	280,119,000	258,609,000
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	167,457,000	128,103,000	323,459,000	252,290,000
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	52,744,000	59,992,000	104,393,000	116,196,000
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	141,193,000	139,517,000	273,363,000	273,750,000
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	72,893,000	34,532,000	138,400,000	65,889,000
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	31,652,000	34,615,000	58,865,000	63,383,000
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	245,738,000	208,664,000	470,628,000	403,022,000
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 120,979,000	$ 115,651,000	$ 237,343,000	$ 224,073,000
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	40.00%	42.00%	40.00%	41.00%
Customer Concentration Risk | Revenue from Contract with Customer | Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	39.00%	46.00%	40.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	14.00%	19.00%	14.00%	19.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	38.00%	43.00%	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	20.00%	11.00%	19.00%	10.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	9.00%	11.00%	8.00%	10.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	67.00%	65.00%	66.00%	64.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	33.00%	35.00%	34.00%	36.00%
Energy Environmental And Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 166,998,000	$ 133,276,000	$ 320,988,000	$ 257,526,000
Energy Environmental And Infrastructure | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	46.00%	41.00%	46.00%	41.00%
Health Education And Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 133,919,000	$ 131,141,000	$ 255,057,000	$ 254,069,000
Health Education And Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	36.00%	41.00%	36.00%	41.00%
Safety And Security
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 29,818,000	$ 27,491,000	$ 59,047,000	$ 52,966,000
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	$ 35,982,000	$ 32,407,000	$ 72,879,000	$ 62,534,000
Consumer And Financial Services | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	10.00%	10.00%	10.00%	10.00%