Earnings Per Share (''EPS'') (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 29, 2014	Jun. 30, 2013	Jun. 29, 2014	Jun. 30, 2013
Number of weighted-average shares used to compute basic and diluted EPS:
Net income (loss) attributable to Tetra Tech	$ 26,657	$ (78,385)	$ 85,680	$ (27,342)
Weighted-average common shares outstanding - basic	64,566,000	64,832,000	64,683,000	64,554,000
Effect of dilutive stock options and unvested restricted stock	736,000		810,000
Weighted-average common stock outstanding - diluted	65,302,000	64,832,000	65,493,000	64,554,000
Earnings (loss) per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.41	$ (1.21)	$ 1.32	$ (0.42)
Diluted (in dollars per share)	$ 0.41	$ (1.21)	$ 1.31	$ (0.42)
Employee and Directors Stock Options [Member]
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares	700,000		0
Potential Common Shares [Member]
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares		700,000		800,000