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Mergers and Acquisitions - Pro-Forma Operating Results (Details) - Coffey and INDUS - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
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Oct. 01, 2017 | Oct. 02, 2016
Pro forma operating results |  |
Revenue | $ 2,753,360 | $ 2,714,658
Operating income | 183,342 | 152,676
Net income attributable to Tetra Tech | $ 117,874 | $ 98,871
Earnings per share attributable to Tetra Tech |  |
Basic (in dollars per share) | $ 2.07 | $ 1.7
Diluted (in dollars per share) | $ 2.04 | $ 1.68

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