Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 30, 2018	Dec. 31, 2017
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	$ 966,971
Other comprehensive (loss) income before reclassifications	(27,117)	$ (3,383)
Interest rate contracts, net of tax	(226)	(18)
Other comprehensive loss attributable to Tetra Tech	(27,343)	(3,401)
Balance at the end of the period	954,156
Foreign Currency Translation Adjustments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(128,602)	(98,946)
Other comprehensive (loss) income before reclassifications	(23,334)	(3,467)
Interest rate contracts, net of tax	0	0
Other comprehensive loss attributable to Tetra Tech	(23,334)	(3,467)
Balance at the end of the period	(151,936)	(102,413)
Gain (Loss) on Derivative Instruments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	1,252	446
Other comprehensive (loss) income before reclassifications	(3,783)	84
Interest rate contracts, net of tax	(226)	(18)
Other comprehensive loss attributable to Tetra Tech	(4,009)	66
Balance at the end of the period	(2,757)	512
Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(127,350)	(98,500)
Balance at the end of the period	$ (154,693)	$ (101,901)