Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Sep. 30, 2012
Current assets:
Cash and cash equivalents	$ 159,575	$ 104,848
Accounts receivable - net	687,120	700,480
Prepaid expenses and other current assets	53,167	48,168
Income taxes receivable	21,294	5,817
Total current assets	921,156	859,313
Property and equipment - net	93,192	74,309
Investments in and advances to unconsolidated joint ventures	3,586	3,279
Goodwill	777,739	635,958
Intangible assets - net	106,859	74,231
Other assets	25,402	23,940
Total assets	1,927,934	1,671,030
Current liabilities:
Accounts payable	148,136	154,003
Accrued compensation	97,881	128,086
Billings in excess of costs on uncompleted contracts	79,131	90,909
Deferred income taxes	19,550	20,809
Current portion of long-term debt	1,730	2,031
Estimated contingent earn-out liabilities	29,699	35,407
Other current liabilities	69,884	72,549
Total current liabilities	446,011	503,794
Deferred income taxes	38,631	24,268
Long-term debt	264,047	81,047
Long-term estimated contingent earn-out liabilities	71,803	16,132
Other long-term liabilities	29,749	25,922
Commitments and contingencies
Equity:
Preferred stock - Authorized, 2,000 shares of $0.01 par value; no shares issued and outstanding at March 31, 2013, and September 30, 2012
Common stock - Authorized, 150,000 shares of $0.01 par value; issued and outstanding, 64,925 and 63,837 shares at March 31, 2013, and September 30, 2012, respectively	649	638
Additional paid-in capital	457,428	433,009
Accumulated other comprehensive income	13,377	31,017
Retained earnings	605,349	554,306
Tetra Tech stockholders' equity	1,076,803	1,018,970
Noncontrolling interests	890	897
Total equity	1,077,693	1,019,867
Total liabilities and equity	$ 1,927,934	$ 1,671,030