Quarterly Financial Information - Unaudited (Details) - USD ($) $ / shares in Units, shares in Thousands		3 Months Ended								12 Months Ended
	Jun. 29, 2020	Sep. 27, 2020	Jun. 28, 2020	Mar. 29, 2020	Dec. 29, 2019	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 30, 2018	Sep. 27, 2020	Sep. 29, 2019	Sep. 30, 2018
Business Acquisition
COVID-19 incremental costs				$ 8,200,000
Impairment of goodwill	$ 0	$ 15,800,000				$ 7,800,000				$ 15,800,000	$ 7,755,000	$ 0
Gain on sale of property and equipment		1,000,000.0	$ 4,500,000	2,200,000	$ 800,000					8,500,000
Severance and other disposition costs						10,900,000
Revenue		753,364,000	709,771,000	734,133,000	797,623,000	841,502,000	$ 825,793,000	$ 722,621,000	$ 717,431,000	2,994,891,000	3,107,348,000	2,964,148,000
Income from operations		66,735,000	63,525,000	47,530,000	63,302,000	20,665,000	64,841,000	47,545,000	55,711,000	241,091,000	188,762,000	190,086,000
Net income attributable to Tetra Tech		$ 44,654,000	$ 45,497,000	$ 36,397,000	$ 47,310,000	$ 11,527,000	$ 49,233,000	$ 55,911,000	$ 41,997,000	$ 173,859,000	$ 158,668,000	$ 136,883,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)		$ 0.83	$ 0.84	$ 0.67	$ 0.87	$ 0.21	$ 0.90	$ 1.01	$ 0.76	$ 3.21	$ 2.89	$ 2.46
Diluted (in dollars per share)		$ 0.82	$ 0.83	$ 0.66	$ 0.85	$ 0.21	$ 0.88	$ 1.00	$ 0.75	$ 3.16	$ 2.84	$ 2.42
Weighted-average common shares outstanding:
Basic (in shares)		53,841	53,985	54,699	54,560	54,617	54,819	55,143	55,390	54,235	54,986	55,670
Diluted (in shares)		54,603	54,692	55,463	55,438	55,618	55,768	55,985	56,366	55,022	55,936	56,598
Foreign
Business Acquisition
Decrease in deferred tax valuation allowance								$ 22,300,000
Contingent consideration
Business Acquisition
Net gains in earnings for adjustments on contingent earn-out liabilities		$ 13,500,000
WYG plc
Business Acquisition
Business Combination, Acquisition Related Costs						$ 10,400,000