Earnings Per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2017	Jan. 01, 2017
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 46,034	$ 26,562
Weighted-average common shares outstanding – basic (in shares)	55,855	57,099
Effect of dilutive stock options and unvested restricted stock (in shares)	1,020	1,046
Weighted-average common stock outstanding – diluted (in shares)	56,875	58,145
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.82	$ 0.47
Diluted (in dollars per share)	$ 0.81	$ 0.46