Earnings Per Share ("EPS") (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Apr. 01, 2012	Mar. 31, 2013	Apr. 01, 2012
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 24,820	$ 22,284	$ 51,043	$ 44,894
Weighted-average common shares outstanding - basic	64,551,000	63,072,000	64,376,000	62,846,000
Effect of diluted stock options and unvested restricted stock	921,000	745,000	832,000	651,000
Weighted-average common stock outstanding - diluted	65,472,000	63,817,000	65,208,000	63,497,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.38	$ 0.35	$ 0.79	$ 0.71
Diluted (in dollars per share)	$ 0.38	$ 0.35	$ 0.78	$ 0.71
Options excluded from the calculation of dilutive potential common shares	400,000	2,400,000	600,000	3,000,000