Earnings Per Share ("EPS") (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Jan. 01, 2012	Jan. 02, 2011
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 22,610	$ 22,301
Weighted-average common shares outstanding - basic	62,433,000	61,665,000
Effect of diluted stock options and unvested restricted stock (in shares)	635,000	778,000
Weighted-average common stock outstanding - diluted (in shares)	63,068,000	62,443,000
Earnings per share attributable to Tetra Tech
Basic (in dollars per share)	$ 0.36	$ 0.36
Diluted (in dollars per share)	$ 0.36	$ 0.36
Options excluded from the calculation of dilutive potential common shares (in shares)	3,400,000	2,900,000