Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
Sep. 29, 2019	Sep. 30, 2018	Oct. 01, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 158,761	$ 136,957	$ 117,917
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments	(21,109)	(29,656)	27,894
(Loss) gain on cash flow hedge valuations	(12,125)	806	1,614
Other comprehensive (loss) income attributable to Tetra Tech	(33,234)	(28,850)	29,508
Other comprehensive income (loss) attributable to noncontrolling interests	243	(64)	8
Comprehensive income	125,770	108,043	147,433
Comprehensive income attributable to Tetra Tech	125,434	108,033	147,382
Comprehensive income attributable to noncontrolling interests	$ 336	$ 10	$ 51

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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