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Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands | 3 Months Ended
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Dec. 30, 2018 | Dec. 31, 2017
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 717,431 | $ 759,749
U.S. state and local government |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 123,280 | 151,754
U.S. federal government |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 224,757 | 236,249
U.S. commercial |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 172,788 | 206,786
International |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 196,606 | $ 164,960
GSG |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 717,431 |
GSG | Fixed-price |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 240,933 |
GSG | Time-and-materials |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 336,537 |
GSG | Cost-plus |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 139,961 |
CIG |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 759,749 |
CIG | Fixed-price |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 235,420 |
CIG | Time-and-materials |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 376,773 |
CIG | Cost-plus |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 147,556 |

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