Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
Jan. 02, 2022	Dec. 27, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 858,510	$ 765,104
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	331,248	274,406
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	395,648	355,270
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	131,614	135,428
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	159,008	125,008
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	266,797	265,873
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	176,904	157,787
International
Disaggregation of Revenue [Line Items]
Revenue	$ 255,801	$ 216,436

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8906-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e9031-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-Publisher FASB
-Name Accounting Standards Codification
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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+ Details