Quarterly Financial Information-Unaudited (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 27, 2015 USD ($) $ / shares shares	Jun. 28, 2015 USD ($) $ / shares shares	Mar. 29, 2015 USD ($) $ / shares shares	Dec. 28, 2014 USD ($) $ / shares shares	Sep. 28, 2014 USD ($) item $ / shares shares	Jun. 29, 2014 USD ($) $ / shares shares	Mar. 30, 2014 USD ($) $ / shares shares	Dec. 29, 2013 USD ($) $ / shares shares	Sep. 27, 2015 USD ($) $ / shares shares	Sep. 28, 2014 USD ($) item $ / shares shares	Sep. 29, 2013 USD ($) $ / shares shares
Quarterly financial information
Impairment charge	$ 60,800								$ 60,800
Impairment charge after tax	57,300
Revenue	578,394	$ 575,108	$ 564,763	$ 581,056	$ 622,179	$ 629,502	$ 586,285	$ 645,848	2,299,321	$ 2,483,814	$ 2,613,755
Operating income (loss)	(20,047)	40,721	30,398	36,612	24,762	39,167	46,186	43,718	87,684	153,833	20,218
Net income (loss) attributable to Tetra Tech	$ (31,724)	$ 26,206	$ 19,017	$ 25,575	$ 22,585	$ 26,657	$ 31,709	$ 27,315	$ 39,074	$ 108,266	$ (2,141)
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share) | $ / shares	$ (0.53)	$ 0.44	$ 0.31	$ 0.41	$ 0.36	$ 0.41	$ 0.49	$ 0.43	$ 0.64	$ 1.68	$ (0.03)
Diluted (in dollars per share) | $ / shares	$ (0.53)	$ 0.43	$ 0.31	$ 0.41	$ 0.35	$ 0.41	$ 0.48	$ 0.42	$ 0.64	$ 1.66	$ (0.03)
Weighted-average common shares outstanding:
Basic (in shares) | shares	59,963	60,207	61,153	62,452	63,602	64,566	64,835	64,227	60,913	64,379	64,544
Diluted (in shares) | shares	59,963	60,792	61,723	63,112	64,235	65,302	65,710	65,048	61,532	65,146	64,544
RCM
Quarterly financial information
Revenue									$ 86,575	$ 221,108	$ 305,821
Operating income (loss)					$ (35,100)				$ (8,614)	$ (45,151)	$ (24,986)
Project charges					$ 25,600
Number of lines of business decided to exit or wind-down | item					2					2
Contingent earn-out liabilities					$ 23,800					$ 23,800