Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
Dec. 29, 2019	Dec. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 797,623	$ 717,431
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	271,055	240,933
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	388,158	336,537
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	138,410	139,961
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	122,356	123,280
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	245,303	224,757
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	182,971	172,788
International
Disaggregation of Revenue [Line Items]
Revenue	$ 246,993	$ 196,606

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 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration