Quarterly Financial Information-Unaudited (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended								12 Months Ended
	Jun. 30, 2014	Sep. 28, 2014	Jun. 29, 2014	Mar. 30, 2014	Dec. 29, 2013	Sep. 29, 2013	Jun. 30, 2013 item	Mar. 31, 2013	Dec. 30, 2012	Sep. 28, 2014	Sep. 29, 2013	Sep. 30, 2012
Quarterly financial information
Number of programs for which project charges recorded							4
Reduction in operating income due to project charge							$ 35,500,000
Reduction in operating income due to charge related to Eastern Canada and global mining operations							28,200,000
Revenue		622,179,000	629,502,000	586,285,000	645,848,000	698,376,000	614,835,000	641,999,000	658,545,000	2,483,814,000	2,613,755,000	2,711,075,000
Operating income (loss)		24,763,000	39,167,000	46,186,000	43,718,000	40,626,000	(99,884,000)	37,667,000	41,809,000	153,833,000	20,218,000	166,367,000
Non-cash goodwill impairment charges	0						56,600,000				56,600,000	914,000
Net income attributable to Tetra Tech		22,586,000	26,657,000	31,709,000	27,315,000	25,200,000	(78,385,000)	24,820,000	26,224,000	108,266,000	(2,141,000)	104,380,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)		$ 0.36	$ 0.41	$ 0.49	$ 0.43	$ 0.39	$ (1.21)	$ 0.38	$ 0.41	$ 1.68	$ (0.03)	$ 1.65
Diluted (in dollars per share)		$ 0.35	$ 0.41	$ 0.48	$ 0.42	$ 0.39	$ (1.21)	$ 0.38	$ 0.41	$ 1.66	$ (0.03)	$ 1.63
Weighted-average common shares outstanding:
Basic (in shares)		63,602	64,566	64,835	64,227	64,272	64,832	64,551	63,864	64,379	64,544	63,217
Diluted (in shares)		64,235	65,302	65,710	65,048	64,853	64,832	65,472	64,608	65,146	64,544	63,934
RCM
Quarterly financial information
Number of programs for which project charges recorded							3
Revenue										703,253,000	725,689,000	621,957,000
Operating income (loss)		(35,100,000)								(34,310,000)	(6,706,000)	22,374,000
Project charges		25,600,000
Number of lines of business decided to exit or wind-down		2								2
Contingent earn-out liabilities		$ 23,800,000								$ 23,800,000