Basis of Presentation and Preparation - Cash and Cash Equivalents and Accounts Receivable (Details)	12 Months Ended
Sep. 27, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Maximum term of original maturity to classify instrument as cash equivalent	90 days
Period for billing and collecting unbilled receivables	12 months

Represents the period within which substantially all unbilled receivables are expected to be billed and collected.

No definition available.

Name:	ttek_UnbilledContractsReceivablePeriodForBillingAndCollection
Namespace Prefix:	ttek_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration