Quarterly Financial Information - Unaudited (Details) - USD ($) $ / shares in Units, shares in Thousands		3 Months Ended								12 Months Ended
	Jul. 01, 2019	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 30, 2018	Sep. 30, 2018	Jul. 01, 2018	Apr. 01, 2018	Dec. 31, 2017	Sep. 29, 2019	Sep. 30, 2018	Oct. 01, 2017
Business acquisition
Deferred tax benefit from tax reform remeasurement									$ 14,700,000
Decrease in revenue related to construction settlement						$ 10,600,000
Loss from construction settlement claim						12,500,000
Decrease in deferred tax valuation allowance				$ 23,400,000
Impairment of goodwill	$ 0	$ 7,800,000								$ 7,755,000	$ 0	$ 0
Severance and other disposition costs		10,900,000
Revenue		841,502,000	$ 825,793,000	722,621,000	$ 717,431,000	739,343,000	$ 764,795,000	$ 700,262,000	759,749,000
Income from operations		20,665,000	64,841,000	47,545,000	55,711,000	43,285,000	55,496,000	42,716,000	48,589,000	188,762,000	190,086,000	183,342,000
Net income attributable to Tetra Tech		$ 11,527,000	$ 49,233,000	$ 55,911,000	$ 41,997,000	$ 28,802,000	$ 33,322,000	$ 28,725,000	$ 46,034,000	$ 158,668,000	$ 136,883,000	$ 117,874,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)		$ 0.21	$ 0.90	$ 1.01	$ 0.76	$ 0.52	$ 0.60	$ 0.51	$ 0.82	$ 2.89	$ 2.46	$ 2.07
Diluted (in dollars per share)		$ 0.21	$ 0.88	$ 1.00	$ 0.75	$ 0.51	$ 0.59	$ 0.51	$ 0.81	$ 2.84	$ 2.42	$ 2.04
Weighted-average common shares outstanding:
Basic (in shares)		54,617	54,819	55,143	55,390	55,341	55,537	55,841	55,855	54,986	55,670	56,911
Diluted (in shares)		55,618	55,768	55,985	56,366	56,349	56,390	56,673	56,875	55,936	56,598	57,913
Disposed of by Sale | Non-core field services business
Business acquisition
Loss on disposition of business							$ 3,400,000				$ 3,400,000
WYG plc
Business acquisition
Acquisition related costs		$ 10,400,000