Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 28, 2021	Mar. 29, 2020	Mar. 28, 2021	Mar. 29, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 45,528	$ 36,413	$ 97,976	$ 83,730
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	10,975	(47,015)	43,368	(33,114)
Gain (loss) on cash flow hedge valuations, net of tax	1,890	(7,365)	3,366	(5,695)
Other comprehensive income (loss), net of tax	12,865	(54,380)	46,734	(38,809)
Comprehensive income (loss), net of tax	58,393	(17,967)	144,710	44,921
Comprehensive income attributable to noncontrolling interests, net of tax	13	12	27	21
Comprehensive income (loss) attributable to Tetra Tech, net of tax	$ 58,380	$ (17,979)	$ 144,683	$ 44,900