Earnings Per Share (''EPS'') (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 30, 2014	Mar. 31, 2013	Mar. 30, 2014	Mar. 31, 2013
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 31,709	$ 24,820	$ 59,023	$ 51,043
Weighted-average common shares outstanding - basic	64,835,000	64,551,000	64,670,000	64,376,000
Effect of dilutive stock options and unvested restricted stock	875,000	921,000	847,000	832,000
Weighted-average common stock outstanding - diluted	65,710,000	65,472,000	65,517,000	65,208,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.49	$ 0.38	$ 0.91	$ 0.79
Diluted (in dollars per share)	$ 0.48	$ 0.38	$ 0.90	$ 0.78
Options
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares	300,000	400,000	0	600,000