Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Oct. 02, 2016
Income Statement [Abstract]
Revenue	$ 2,964,148	$ 2,753,360	$ 2,583,469
Subcontractor costs	(763,414)	(719,350)	(654,264)
Other costs of revenue	(1,816,276)	(1,680,372)	(1,598,994)
Gross profit	384,458	353,638	330,211
Selling, general and administrative expenses	(190,120)	(177,219)	(171,985)
Acquisition and integration expenses	0	0	(19,548)
Contingent consideration – fair value adjustments	(4,252)	6,923	(2,823)
Income from operations	190,086	183,342	135,855
Interest income	1,824	729	996
Interest expense	(17,348)	(12,310)	(12,385)
Income before income tax expense	174,562	171,761	124,466
Income tax expense	(37,605)	(53,844)	(40,613)
Net income	136,957	117,917	83,853
Net income attributable to noncontrolling interests	(74)	(43)	(70)
Net income attributable to Tetra Tech	$ 136,883	$ 117,874	$ 83,783
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 2.46	$ 2.07	$ 1.44
Diluted (in dollars per share)	$ 2.42	$ 2.04	$ 1.42
Weighted-average common shares outstanding:
Basic (in shares)	55,670	56,911	58,186
Diluted (in shares)	56,598	57,913	58,966
Cash dividends paid per share (in dollars per share)	$ 0.44	$ 0.38	$ 0.34