Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jul. 01, 2018	Jul. 02, 2017	Jul. 01, 2018	Jul. 02, 2017
Income Statement [Abstract]
Revenue	$ 764,795	$ 685,539	$ 2,224,805	$ 2,018,171
Subcontractor costs	(194,443)	(187,061)	(576,813)	(518,188)
Other costs of revenue	(460,758)	(408,228)	(1,352,827)	(1,247,369)
Gross profit	109,594	90,250	295,165	252,614
Selling, general and administrative expenses	(53,906)	(44,366)	(146,254)	(131,068)
Contingent consideration – fair value adjustments	(192)	0	(2,110)	7,149
Income from operations	55,496	45,884	146,801	128,695
Interest expense	(4,345)	(2,795)	(11,597)	(8,802)
Income before income tax expense	51,151	43,089	135,204	119,893
Income tax expense	(17,806)	(13,114)	(27,060)	(36,462)
Net income	33,345	29,975	108,144	83,431
Net (income) loss attributable to noncontrolling interests	(23)	8	(62)	(24)
Net income attributable to Tetra Tech	$ 33,322	$ 29,983	$ 108,082	$ 83,407
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.60	$ 0.52	$ 1.94	$ 1.46
Diluted (in dollars per share)	$ 0.59	$ 0.52	$ 1.91	$ 1.44
Weighted-average common shares outstanding:
Basic (in shares)	55,537	57,184	55,780	57,108
Diluted (in shares)	56,390	58,161	56,681	58,116
Cash dividends paid per share (in dollars per share)	$ 0.12	$ 0.1	$ 0.32	$ 0.28