Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2019	Apr. 01, 2018	Mar. 31, 2019	Apr. 01, 2018
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period			$ 966,971
Other comprehensive loss before reclassifications	$ 6,741	$ (16,252)	(20,376)	$ (19,635)
Interest rate contracts, net of tax	(265)	109	(491)	91
Other comprehensive income (loss) attributable to Tetra Tech	6,476	(16,143)	(20,867)	(19,544)
Balance at the end of the period	990,445		990,445
Foreign Currency Translation Adjustments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(151,936)	(102,413)	(128,602)	(98,946)
Other comprehensive loss before reclassifications	9,245	(16,116)	(14,089)	(19,583)
Interest rate contracts, net of tax	0	0	0	0
Other comprehensive income (loss) attributable to Tetra Tech	9,245	(16,116)	(14,089)	(19,583)
Balance at the end of the period	(142,691)	(118,529)	(142,691)	(118,529)
Gain (Loss) on Derivative Instruments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(2,757)	512	1,252	446
Other comprehensive loss before reclassifications	(2,504)	(136)	(6,287)	(52)
Interest rate contracts, net of tax	(265)	109	(491)	91
Other comprehensive income (loss) attributable to Tetra Tech	(2,769)	(27)	(6,778)	39
Balance at the end of the period	(5,526)	485	(5,526)	485
Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(154,693)	(101,901)	(127,350)	(98,500)
Balance at the end of the period	$ (148,217)	$ (118,044)	$ (148,217)	$ (118,044)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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