Revenue Recognition - Income Statement Impact of Adoption of ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2019	Apr. 01, 2018	Mar. 31, 2019	Apr. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 722,621	$ 700,262	$ 1,440,052	$ 1,460,010
Income from operations	47,545	42,716	103,256	91,305
Income tax benefit (expense)	11,563	(9,877)	781	(9,254)
Net income attributable to Tetra Tech	55,911	$ 28,725	97,907	$ 74,759
Recognition Under Previous Guidance
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	723,666		1,438,894
Income from operations	48,590		102,098
Income tax benefit (expense)	11,317		1,061
Net income attributable to Tetra Tech	56,710		97,028
Impact of the Adoption of ASC 606 \| Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	(1,045)		1,158
Income from operations	(1,045)		1,158
Income tax benefit (expense)	246		(280)
Net income attributable to Tetra Tech	$ (799)		$ 879

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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