Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2018	Oct. 01, 2017	Oct. 02, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 136,957	$ 117,917	$ 83,853
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments	(29,656)	27,894	14,389
Gain on cash flow hedge valuations	806	1,614	774
Other comprehensive income (loss) attributable to Tetra Tech	(28,850)	29,508	15,163
Other comprehensive income (loss) attributable to noncontrolling interests	(64)	8	3
Comprehensive income	108,043	147,433	99,019
Comprehensive income attributable to Tetra Tech	108,033	147,382	98,946
Comprehensive income attributable to noncontrolling interests	$ 10	$ 51	$ 73

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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