Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jul. 01, 2018	Jun. 30, 2019	Jul. 01, 2018
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period			$ 966,971
Other comprehensive loss before reclassifications	$ 1,331	$ (13,388)	(19,045)	$ (33,023)
Interest rate contracts, net of tax	(192)	(306)	(683)	(215)
Other comprehensive income (loss) attributable to Tetra Tech	1,139	(13,694)	(19,728)	(33,238)
Balance at the end of the period	1,015,612		1,015,612
Foreign Currency Translation Adjustments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(142,691)	(118,529)	(128,602)	(98,946)
Other comprehensive loss before reclassifications	5,428	(13,209)	(8,661)	(32,792)
Interest rate contracts, net of tax	0	0	0	0
Other comprehensive income (loss) attributable to Tetra Tech	5,428	(13,209)	(8,661)	(32,792)
Balance at the end of the period	(137,263)	(131,738)	(137,263)	(131,738)
Gain (Loss) on Derivative Instruments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(5,526)	485	1,252	446
Other comprehensive loss before reclassifications	(4,097)	(179)	(10,384)	(231)
Interest rate contracts, net of tax	(192)	(306)	(683)	(215)
Other comprehensive income (loss) attributable to Tetra Tech	(4,289)	(485)	(11,067)	(446)
Balance at the end of the period	(9,815)	0	(9,815)	0
Accumulated Other Comprehensive Gain (Loss)
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(148,217)	(118,044)	(127,350)	(98,500)
Balance at the end of the period	$ (147,078)	$ (131,738)	$ (147,078)	$ (131,738)