Consolidated Statements of Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests
Beginning balance (in shares) at Sep. 30, 2018		55,349
Beginning balance at Sep. 30, 2018	$ 967,100	$ 553	$ 148,803	$ (127,350)	$ 944,965	$ 966,971	$ 129
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42,032				41,997	41,997	35
Other comprehensive income (loss)	(27,106)			(27,343)		(27,343)	237
Distributions paid to noncontrolling interests	(254)						(254)
Cash dividends per common share	(6,654)				(6,654)	(6,654)
Stock-based compensation	4,529		4,529			4,529
Restricted & performance shares released (in shares)		174
Restricted & performance shares released	(6,740)	$ 2	(6,742)			(6,740)
Stock options exercised (in shares)		86
Stock options exercised	2,314	$ 1	2,313			2,314
Shares issued for Employee Stock Purchase Plan (in shares)		148
Shares issued for Employee Stock Purchase Plan	6,847	$ 1	6,846			6,847
Stock repurchases (in shares)		(431)
Stock repurchases	(25,000)	$ (4)	(24,996)			(25,000)
Ending balance (in shares) at Dec. 30, 2018		55,326
Ending balance at Dec. 30, 2018	954,303	$ 553	130,753	(154,693)	977,543	954,156	147
Beginning balance (in shares) at Sep. 29, 2019		54,565
Beginning balance at Sep. 29, 2019	989,464	$ 546	78,132	(160,584)	1,071,192	989,286	178
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47,317				47,310	47,310	7
Other comprehensive income (loss)	15,571			15,569		15,569	2
Distributions paid to noncontrolling interests	(13)						(13)
Cash dividends per common share	(8,190)				(8,190)	(8,190)
Stock-based compensation	4,482		4,482			4,482
Restricted & performance shares released (in shares)		185
Restricted & performance shares released	(10,818)	$ 1	(10,819)			(10,818)
Stock options exercised (in shares)		54
Stock options exercised	1,413	$ 1	1,412			1,413
Shares issued for Employee Stock Purchase Plan (in shares)		168
Shares issued for Employee Stock Purchase Plan	8,716	$ 1	8,715			8,716
Stock repurchases (in shares)		(244)
Stock repurchases	(21,177)	$ (2)	(21,175)			(21,177)
Ending balance (in shares) at Dec. 29, 2019		54,728
Ending balance at Dec. 29, 2019	$ 1,026,765	$ 547	$ 60,747	$ (145,015)	$ 1,110,312	$ 1,026,591	$ 174