Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Apr. 02, 2023	Mar. 31, 2024	Apr. 02, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 76,459	$ 42,841	$ 151,439	$ 159,556
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	(24,344)	(8,154)	38,762	24,953
Loss on cash flow hedge valuations, net of tax	0	(896)	0	(985)
Net pension adjustments	0	2,794	(13)	2,794
Other comprehensive income (loss), net of tax	(24,344)	(6,256)	38,749	26,762
Comprehensive income, net of tax	52,115	36,585	190,188	186,318
Comprehensive income attributable to noncontrolling interests, net of tax	13	10	21	19
Comprehensive income attributable to Tetra Tech, net of tax	$ 52,102	$ 36,575	$ 190,167	$ 186,299