Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Apr. 01, 2018	Mar. 31, 2019	Apr. 01, 2018
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 55,911	$ 28,725	$ 97,907	$ 74,759
Weighted-average common shares outstanding – basic (in shares)	55,143	55,841	55,237	55,900
Effect of dilutive stock options and unvested restricted stock (in shares)	842	832	924	925
Weighted-average common stock outstanding – diluted (in shares)	55,985	56,673	56,161	56,825
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.01	$ 0.51	$ 1.77	$ 1.34
Diluted (in dollars per share)	$ 1.00	$ 0.51	$ 1.74	$ 1.32