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Revenue Recognition - Income Statement Impact of Adoption of ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 30, 2018	Sep. 30, 2018	Jul. 01, 2018	Apr. 01, 2018	Dec. 31, 2017	Sep. 29, 2019	Sep. 30, 2018	Oct. 01, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue									$ 3,107,348	$ 2,964,148	$ 2,753,360
Income from operations	$ 20,665	$ 64,841	$ 47,545	$ 55,711	$ 43,285	$ 55,496	$ 42,716	$ 48,589	188,762	190,086	183,342
Income tax expense									(16,375)	(37,605)	(53,844)
Net income attributable to Tetra Tech	$ 11,527	$ 49,233	$ 55,911	$ 41,997	$ 28,802	$ 33,322	$ 28,725	$ 46,034	158,668	$ 136,883	$ 117,874
Recognition Under Previous Guidance
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue									3,105,621
Income from operations									187,035
Income tax expense									(15,874)
Net income attributable to Tetra Tech									157,442
Impact of the Adoption of ASC 606 \| Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue									1,727
Income from operations									1,727
Income tax expense									(501)
Net income attributable to Tetra Tech									$ 1,226