Joint Ventures (Details) (USD $)	3 Months Ended		9 Months Ended
	Jul. 03, 2011	Jun. 27, 2010	Jul. 03, 2011	Jun. 27, 2010	Oct. 03, 2010
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	$ 22,109,000		$ 22,109,000		$ 17,036,000
TOTAL ASSETS	1,571,827,000		1,571,827,000		1,381,689,000
Accounts payable	184,391,000		184,391,000		166,450,000
Total Tetra Tech equity	853,944,000		853,944,000		748,133,000
Noncontrolling interests	14,161,000		14,161,000
TOTAL STOCKHOLDERS' EQUITY	868,105,000		868,105,000		748,133,000
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	1,571,827,000		1,571,827,000		1,381,689,000
Aggregate revenue of consolidated joint ventures	673,792,000	562,365,000	1,897,482,000	1,573,850,000
Unconsolidated Joint Ventures
Equity in earnings from unconsolidated joint ventures	1,000,000	300,000	3,400,000	900,000
Carrying value of assets of unconsolidated VIEs	28,100,000		28,100,000
Carrying value of liabilities of unconsolidated VIEs	25,200,000		25,200,000
Consolidated Joint Ventures
Joint Ventures
Cash	4,056,000		4,056,000
Other current assets	20,828,000		20,828,000
Non-current assets	16,582,000		16,582,000
TOTAL ASSETS	41,466,000		41,466,000
Accounts payable	3,439,000		3,439,000
Other liabilities	9,700,000		9,700,000
Total liabilities	13,139,000		13,139,000
Total Tetra Tech equity	14,168,000		14,168,000
Noncontrolling interests	14,159,000		14,159,000
TOTAL STOCKHOLDERS' EQUITY	28,327,000		28,327,000
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	41,466,000		41,466,000
Aggregate revenue of consolidated joint ventures	$ 22,400,000		$ 60,800,000