Quarterly Financial Information-Unaudited (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jul. 01, 2018	Apr. 01, 2018	Dec. 31, 2017	Oct. 01, 2017	Jul. 02, 2017	Apr. 02, 2017	Jan. 01, 2017	Sep. 30, 2018	Oct. 01, 2017	Oct. 02, 2016
Business acquisition
Deferred tax benefit from tax reform remeasurement				$ 14,700					$ 14,700
Decrease in revenue related to construction settlement	$ 10,600
Loss from construction settlement claim	12,500
Revenue	739,343	$ 764,795	$ 700,262	759,749	$ 735,188	$ 685,539	$ 663,781	$ 668,851	2,964,148	$ 2,753,360	$ 2,583,469
Income from operations	43,285	55,496	42,716	48,589	54,647	45,884	42,956	39,855	190,086	183,342	135,855
Net income attributable to Tetra Tech	$ 28,802	$ 33,322	$ 28,725	$ 46,034	$ 34,467	$ 29,983	$ 26,862	$ 26,562	$ 136,883	$ 117,874	$ 83,783
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.52	$ 0.60	$ 0.51	$ 0.82	$ 0.61	$ 0.52	$ 0.47	$ 0.47	$ 2.46	$ 2.07	$ 1.44
Diluted (in dollars per share)	$ 0.51	$ 0.59	$ 0.51	$ 0.81	$ 0.60	$ 0.52	$ 0.46	$ 0.46	$ 2.42	$ 2.04	$ 1.42
Weighted-average common shares outstanding:
Basic (in shares)	55,341	55,537	55,841	55,855	56,338	57,184	57,270	57,099	55,670	56,911	58,186
Diluted (in shares)	56,349	56,390	56,673	56,875	57,326	58,161	58,270	58,145	56,598	57,913	58,966
Disposed of by Sale | Non-core field services business
Business acquisition
Loss on disposition of business		$ 3,400							$ 3,400