Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Jan. 01, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 46,052	$ 26,589
Foreign currency translation adjustments	(3,469)	(15,999)
Gain on cash flow hedge valuations	66	996
Other comprehensive loss, net of tax	(3,403)	(15,003)
Comprehensive income, net of tax	42,649	11,586
Net income attributable to noncontrolling interests	(18)	(27)
Foreign currency translation adjustments	2	(183)
Comprehensive income attributable to noncontrolling interests	(16)	(210)
Comprehensive income attributable to Tetra Tech, net of tax	$ 42,633	$ 11,376