Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Oct. 02, 2016
Reclassifications out of accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ 928,453
Other comprehensive income (loss) before reclassifications	(28,441)	$ 30,257
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	(409)	(749)
Other comprehensive income (loss) attributable to Tetra Tech	(28,850)	29,508	$ 15,163
Balance at the end of the period	966,971	928,453
Foreign Currency Translation Adjustments
Reclassifications out of accumulated other comprehensive income (loss)
Balance at the beginning of the period	(98,946)	(126,840)
Other comprehensive income (loss) before reclassifications	(29,656)	27,894
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss) attributable to Tetra Tech	(29,656)	27,894
Balance at the end of the period	(128,602)	(98,946)	(126,840)
Gain (Loss) on Derivative Instruments
Reclassifications out of accumulated other comprehensive income (loss)
Balance at the beginning of the period	446	(1,168)
Other comprehensive income (loss) before reclassifications	1,215	2,363
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	(409)	(749)
Other comprehensive income (loss) attributable to Tetra Tech	806	1,614
Balance at the end of the period	1,252	446	(1,168)
Accumulated Other Comprehensive Income (Loss)
Reclassifications out of accumulated other comprehensive income (loss)
Balance at the beginning of the period	(98,500)	(128,008)
Amounts reclassified from accumulated other comprehensive income
Balance at the end of the period	$ (127,350)	$ (98,500)	$ (128,008)