Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 377,910	$ 332,968	$ 324,315	$ 302,780	$ 321,174	$ 305,301	$ 306,392	$ 296,295	$ 1,337,973	$ 1,229,162	$ 1,185,097
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients									581,965	529,606	511,747
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients									526,728	480,584	456,065
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients									229,280	218,972	217,285
U S Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									543,918	550,794	563,498
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									185,130	127,797	132,287
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									122,293	91,318	75,636
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									851,341	769,909	771,421
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients									486,632	459,253	413,676
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients									565,125	487,001	457,992
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients									535,314	518,675	508,903
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients									111,072	102,645	98,358
Consumer and Financial
Disaggregation Of Revenue [Line Items]
Revenue from clients									$ 126,462	$ 120,841	$ 119,844