Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Income Statement [Abstract]
Revenue	$ 700,262	$ 663,781	$ 1,460,010	$ 1,332,632
Subcontractor costs	(167,469)	(151,827)	(382,370)	(331,127)
Other costs of revenue	(441,368)	(430,952)	(892,070)	(839,141)
Gross profit	91,425	81,002	185,570	162,364
Selling, general and administrative expenses	(46,791)	(45,195)	(92,347)	(86,702)
Contingent consideration – fair value adjustments	(1,918)	7,149	(1,918)	7,149
Income from operations	42,716	42,956	91,305	82,811
Interest expense	(4,092)	(3,099)	(7,252)	(6,007)
Income before income tax expense	38,624	39,857	84,053	76,804
Income tax expense	(9,877)	(12,990)	(9,254)	(23,348)
Net income	28,747	26,867	74,799	53,456
Net income attributable to noncontrolling interests	(22)	(5)	(40)	(32)
Net income attributable to Tetra Tech	$ 28,725	$ 26,862	$ 74,759	$ 53,424
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.51	$ 0.47	$ 1.34	$ 0.93
Diluted (in dollars per share)	$ 0.51	$ 0.46	$ 1.32	$ 0.92
Weighted-average common shares outstanding:
Basic (in shares)	55,841	57,270	55,900	57,171
Diluted (in shares)	56,673	58,270	56,825	58,194
Cash dividends paid per share (in dollars per share)	$ 0.1	$ 0.09	$ 0.2	$ 0.18