Revenue Recognition - Income Statement Impact of Adoption of ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended
Dec. 30, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 717,431	$ 759,749
Income from operations	55,711	48,589
Income tax (expense) benefit	10,782	(623)
Net income attributable to Tetra Tech	41,997	$ 46,034
Recognition Under Previous Guidance
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	715,228
Income from operations	53,508
Income tax (expense) benefit	(10,257)
Net income attributable to Tetra Tech	40,319
Impact of the Adoption of ASC 606 \| Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	2,203
Income from operations	2,203
Income tax (expense) benefit	(525)
Net income attributable to Tetra Tech	$ 1,678

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

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