Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 28, 2014	Jun. 29, 2014	Mar. 30, 2014	Dec. 29, 2013	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 28, 2014	Sep. 29, 2013	Sep. 30, 2012
Number of weighted-average shares used to compute basic and diluted EPS:
Net income (loss) attributable to Tetra Tech	$ 22,586	$ 26,657	$ 31,709	$ 27,315	$ 25,200	$ (78,385)	$ 24,820	$ 26,224	$ 108,266	$ (2,141)	$ 104,380
Weighted-average common shares outstanding - basic	63,602,000	64,566,000	64,835,000	64,227,000	64,272,000	64,832,000	64,551,000	63,864,000	64,379,000	64,544,000	63,217,000
Effect of diluted stock options and unvested restricted stock									767,000		717,000
Weighted-average common stock outstanding - diluted	64,235,000	65,302,000	65,710,000	65,048,000	64,853,000	64,832,000	65,472,000	64,608,000	65,146,000	64,544,000	63,934,000
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
Stock options
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares									0		1,900,000
Potential common shares
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares										500,000