Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 332,968	$ 305,301	$ 960,063	$ 907,988
Fixed-Price [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	126,332		384,870
Time-and-Materials [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	146,472		399,023
Cost-Based [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	60,164		176,170
U S Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	139,875		412,403
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	55,828		122,958
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	26,559		89,968
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	222,262		625,329
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	110,706		334,734
Energy, Environmental and Infrastructure [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	147,840		404,476
Health, Education and Social Programs [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	126,347		381,287
Safety and Security [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	29,446		82,412
Consumer and Financial [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 29,335		$ 91,888