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Mergers and Acquisitions - Pro-Forma Operating Results (Details) - Coffey and INDUS - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended
Jul. 02, 2017
Jun. 26, 2016
Mergers and Acquisitions    
Revenue $ 2,018,171 $ 1,986,149
Operating income 128,695 102,855
Net income attributable to Tetra Tech $ 83,407 $ 65,923
Earnings per share attributable to Tetra Tech, Basic (in dollars per share) $ 1.46 $ 1.12
Earnings per share attributable to Tetra Tech, Diluted (in dollars per share) $ 1.44 $ 1.11