Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Jan. 01, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 74,980	$ 116,715
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	63,106	33,107
Gain (loss) on cash flow hedge valuations, net of tax	0	(89)
Net pension adjustments	(13)	0
Other comprehensive income, net of tax	63,093	33,018
Comprehensive income, net of tax	138,073	149,733
Comprehensive income attributable to noncontrolling interests, net of tax	8	9
Comprehensive income attributable to Tetra Tech, net of tax	$ 138,065	$ 149,724