Basis of Presentation and Preparation - Revenue Recognition and Contract Costs (Details)	12 Months Ended
Sep. 30, 2018 Contract
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of types of contracts under which revenue is recognized	3

Represents the number of types of contracts under which revenue is recognized for work performed.

No definition available.

Name:	ttek_NumberOfTypesOfContractsForRevenueRecognition
Namespace Prefix:	ttek_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration