Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 396,636	$ 373,918	$ 366,717	$ 341,254	$ 377,910	$ 332,968	$ 324,315	$ 302,780	$ 1,478,525	$ 1,337,973	$ 1,229,162
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients									703,467	581,446	529,606
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients									562,985	526,751	480,584
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients									212,073	229,776	218,972
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									561,022	546,050	550,794
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									280,357	183,900	127,797
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									122,307	122,186	91,318
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									963,686	852,136	769,909
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients									514,839	485,837	459,253
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients									665,185	564,736	487,001
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients									552,600	535,578	518,675
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients									120,078	111,660	102,645
Consumer and Financial
Disaggregation Of Revenue [Line Items]
Revenue from clients									$ 140,662	$ 125,999	$ 120,841