Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2019	Apr. 01, 2018	Mar. 31, 2019	Apr. 01, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 55,944	$ 28,747	$ 97,976	$ 74,799
Other comprehensive income, net of tax
Foreign currency translation adjustments	9,245	(16,116)	(14,089)	(19,583)
(Loss) gain on cash flow hedge valuations	(2,769)	(27)	(6,778)	39
Other comprehensive income (loss) attributable to Tetra Tech	6,476	(16,143)	(20,867)	(19,544)
Other comprehensive income (loss) attributable to noncontrolling interests	1	(5)	238	(7)
Comprehensive income attributable to Tetra Tech	62,387	12,582	77,040	55,215
Comprehensive income attributable to noncontrolling interests	34	17	307	33
Comprehensive income	$ 62,421	$ 12,599	$ 77,347	$ 55,248

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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