Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 434,335	$ 360,315	$ 353,987	$ 358,238	$ 396,636	$ 373,918	$ 366,717	$ 341,254	$ 1,506,875	$ 1,478,525	$ 1,337,973
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients									749,844	700,980	581,446
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients									529,157	566,299	526,751
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients									227,874	211,246	229,776
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									666,961	560,953	546,050
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									222,730	279,833	183,900
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									95,734	122,125	122,186
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients									985,425	962,911	852,136
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients									521,450	515,614	485,837
Energy, Environmental and Infrastructure
Disaggregation Of Revenue [Line Items]
Revenue from clients									616,296	663,799	564,736
Health, Education and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients									670,618	567,351	535,578
Safety and Security
Disaggregation Of Revenue [Line Items]
Revenue from clients									117,979	118,279	111,660
Consumer and Financial
Disaggregation Of Revenue [Line Items]
Revenue from clients									$ 101,982	$ 129,096	$ 125,999