Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 29, 2019	Sep. 30, 2018	Oct. 01, 2017
Reclassifications out of accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ 966,971
Other comprehensive loss before reclassifications	(32,356)	$ (28,441)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	(878)	(409)
Other comprehensive (loss) income attributable to Tetra Tech	(33,234)	(28,850)	$ 29,508
Balance at the end of the period	989,286	966,971
Foreign Currency Translation Adjustments
Reclassifications out of accumulated other comprehensive income (loss)
Balance at the beginning of the period	(128,602)	(98,946)
Other comprehensive loss before reclassifications	(21,109)	(29,656)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive (loss) income attributable to Tetra Tech	(21,109)	(29,656)
Balance at the end of the period	(149,711)	(128,602)	(98,946)
Gain (Loss) on Derivative Instruments
Reclassifications out of accumulated other comprehensive income (loss)
Balance at the beginning of the period	1,252	446
Other comprehensive loss before reclassifications	(11,247)	1,215
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	(878)	(409)
Other comprehensive (loss) income attributable to Tetra Tech	(12,125)	806
Balance at the end of the period	(10,873)	1,252	446
Accumulated Other Comprehensive Income (Loss)
Reclassifications out of accumulated other comprehensive income (loss)
Balance at the beginning of the period	(127,350)	(98,500)
Amounts reclassified from accumulated other comprehensive income
Balance at the end of the period	$ (160,584)	$ (127,350)	$ (98,500)