Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 30, 2025	Mar. 31, 2024	Mar. 30, 2025	Mar. 31, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 5,412	$ 76,459	$ 6,190	$ 151,439
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	34,574	(24,344)	(74,272)	38,762
Net pension adjustments	0	0	(33)	(13)
Other comprehensive income (loss), net of tax	34,574	(24,344)	(74,305)	38,749
Comprehensive income (loss), net of tax	39,986	52,115	(68,115)	190,188
Less: Comprehensive income attributable to noncontrolling interests, net of tax	24	13	55	21
Comprehensive income (loss) attributable to Tetra Tech, net of tax	$ 39,962	$ 52,102	$ (68,170)	$ 190,167