Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jul. 01, 2018	Jul. 02, 2017	Jul. 01, 2018	Jul. 02, 2017
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 33,322	$ 29,983	$ 108,082	$ 83,407
Weighted-average common shares outstanding – basic (in shares)	55,537	57,184	55,780	57,108
Effect of dilutive stock options and unvested restricted stock (in shares)	853	977	901	1,008
Weighted-average common stock outstanding – diluted (in shares)	56,390	58,161	56,681	58,116
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.60	$ 0.52	$ 1.94	$ 1.46
Diluted (in dollars per share)	$ 0.59	$ 0.52	$ 1.91	$ 1.44