Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 30, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 41,997	$ 46,034
Weighted-average common shares outstanding – basic (in shares)	55,390	55,855
Effect of dilutive stock options and unvested restricted stock (in shares)	976	1,020
Weighted-average common stock outstanding – diluted (in shares)	56,366	56,875
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.76	$ 0.82
Diluted (in dollars per share)	$ 0.75	$ 0.81