Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 45,503	$ 49,251	$ 129,234	$ 147,227
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	21,689	5,428	(11,422)	(8,661)
Loss on cash flow hedge valuations, net of tax	(30)		(5,726)
Loss on cash flow hedge valuations, net of tax		(4,289)		(11,067)
Other comprehensive income (loss) attributable to Tetra Tech, net of tax	21,659	1,139	(17,148)	(19,728)
Other comprehensive income (loss) attributable to noncontrolling interests, net of tax	(4)	3	(6)	242
Comprehensive income attributable to Tetra Tech, net of tax	67,156	50,372	112,057	127,413
Comprehensive income attributable to noncontrolling interests, net of tax	2	21	23	328
Comprehensive income, net of tax	$ 67,158	$ 50,393	$ 112,080	$ 127,741