Basis of Presentation and Preparation - Goodwill and Intangible Assets (Details)	12 Months Ended
Oct. 01, 2017 Level Step
Goodwill and Intangible Assets
Number of levels below reportable segments at which goodwill impairment testing is performed \| Level	1
Number of steps involved in process of goodwill annual impairment test \| Step	2

Represents the number of levels below reportable segments at which the reporting units reside for which goodwill impairment testing is performed.

No definition available.

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