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Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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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

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