Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jul. 01, 2018	Jul. 02, 2017	Jul. 01, 2018	Jul. 02, 2017
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period			$ 928,453
Other comprehensive income (loss) before reclassifications	$ (13,388)	$ 14,321	(33,023)	$ 9,489
Amounts reclassified from accumulated other comprehensive income	(306)	(134)	(215)	(713)
Other comprehensive income (loss) attributable to Tetra Tech	(13,694)	14,187	(33,238)	8,776
Balance at the end of the period	936,091		936,091
Foreign Currency Translation Adjustments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(118,529)	(133,703)	(98,946)	(126,844)
Other comprehensive income (loss) before reclassifications	(13,209)	14,049	(32,792)	7,190
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Other comprehensive income (loss) attributable to Tetra Tech	(13,209)	14,049	(32,792)	7,190
Balance at the end of the period	(131,738)	(119,654)	(131,738)	(119,654)
Gain (Loss) on Derivative Instruments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	485	284	446	(1,164)
Other comprehensive income (loss) before reclassifications	(179)	272	(231)	2,299
Amounts reclassified from accumulated other comprehensive income	(306)	(134)	(215)	(713)
Other comprehensive income (loss) attributable to Tetra Tech	(485)	138	(446)	1,586
Balance at the end of the period	0	422	0	422
Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(118,044)	(133,419)	(98,500)	(128,008)
Balance at the end of the period	$ (131,738)	$ (119,232)	$ (131,738)	$ (119,232)

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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