Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
Dec. 30, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 42,032	$ 46,052
Other comprehensive income, net of tax
Foreign currency translation adjustments	(23,334)	(3,467)
(Loss) gain on cash flow hedge valuations	(4,009)	66
Other comprehensive loss attributable to Tetra Tech	(27,343)	(3,401)
Other comprehensive income (loss) attributable to noncontrolling interests	237	(2)
Comprehensive income	14,926	42,649
Comprehensive income attributable to Tetra Tech	14,654	42,633
Comprehensive income attributable to noncontrolling interests	$ 272	$ 16

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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