Basis of Presentation and Preparation - Goodwill and Intangible Assets (Details)	12 Months Ended
Sep. 30, 2018 Level
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of levels below reportable segments at which goodwill impairment testing is performed	1

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