Revenue Recognition - Income Statement Impact of Adoption of ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jul. 01, 2018	Jun. 30, 2019	Jul. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 825,793	$ 764,795	$ 2,265,846	$ 2,224,805
Income from operations	64,841	55,496	168,097	146,801
Income tax expense	(12,044)	(17,806)	(11,263)	(27,060)
Net income attributable to Tetra Tech	49,233	$ 33,322	147,141	$ 108,082
Recognition Under Previous Guidance
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	826,292		2,265,186
Income from operations	65,340		167,437
Income tax expense	(12,142)		(11,081)
Net income attributable to Tetra Tech	49,634		146,663
Impact of the Adoption of ASC 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	(499)		660
Income from operations	(499)		660
Income tax expense	98		(182)
Net income attributable to Tetra Tech	$ (401)		$ 478