Earnings Per Share (''EPS'') (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Dec. 29, 2013	Dec. 30, 2012
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 27,315	$ 26,224
Weighted-average common shares outstanding - basic	64,227,000	63,864,000
Effect of dilutive stock options and unvested restricted stock	821,000	744,000
Weighted-average common stock outstanding - diluted	65,048,000	64,608,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.43	$ 0.41
Diluted (in dollars per share)	$ 0.42	$ 0.41
Options
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares	400,000	1,400,000