Joint Ventures (Details) (USD $)	3 Months Ended								12 Months Ended
	Oct. 02, 2011	Jul. 03, 2011	Apr. 03, 2011	Jan. 02, 2011	Oct. 03, 2010	Jun. 27, 2010	Mar. 28, 2010	Dec. 27, 2009	Oct. 02, 2011 Y level D entity M step contract W	Oct. 03, 2010	Sep. 27, 2009	Sep. 28, 2008
Joint Ventures
Number of groups into which joint ventures are classified									2
Minimum number of joint venture parties directing VIE activities for no primary beneficiary or consolidation									2
Non-current assets	$ 23,429,000				$ 17,036,000				$ 23,429,000	$ 17,036,000
TOTAL ASSETS	1,593,988,000				1,381,689,000				1,593,988,000	1,381,689,000	1,097,905,000
Accounts payable	164,819,000				166,450,000				164,819,000	166,450,000
Total Tetra Tech equity	854,725,000				748,133,000				854,725,000	748,133,000
Noncontrolling interests	525,000								525,000
TOTAL EQUITY	855,250,000				748,133,000				855,250,000	748,133,000	646,478,000	511,514,000
TOTAL LIABILITIES AND EQUITY	1,593,988,000				1,381,689,000				1,593,988,000	1,381,689,000
Aggregate revenue of consolidated joint ventures	675,662,000	673,792,000	612,566,000	611,124,000	627,382,000	562,365,000	469,528,000	541,957,000	2,573,144,000	2,201,232,000	2,287,484,000
Unconsolidated Joint Ventures
Equity in earnings from unconsolidated joint ventures									4,877,000	1,184,000	1,613,000
Carrying value of assets of unconsolidated VIEs	24,000,000								24,000,000
Carrying value of liabilities of unconsolidated VIEs	21,000,000								21,000,000
Consolidated Joint Ventures
Joint Ventures
Cash	978,000								978,000
Other current assets	2,318,000								2,318,000
Non-current assets	1,000								1,000
TOTAL ASSETS	3,297,000								3,297,000
Accounts payable	2,230,000								2,230,000
Other liabilities	30,000								30,000
Total liabilities	2,260,000								2,260,000
Total Tetra Tech equity	512,000								512,000
Noncontrolling interests	525,000								525,000
TOTAL EQUITY	1,037,000								1,037,000
TOTAL LIABILITIES AND EQUITY	3,297,000								3,297,000
Aggregate revenue of consolidated joint ventures									$ 74,300,000