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 at Sep. 27, 2015	$ 856,798	$ 594	$ 326,593	$ (143,171)	$ 672,309	$ 856,325	$ 473
Beginning balance (in shares) at Sep. 27, 2015		59,381
Comprehensive income, net of tax:
Net income	83,853				83,783	83,783	70
Foreign currency translation adjustments	14,392			14,389		14,389	3
Gain on cash flow hedge valuations	774			774		774
Comprehensive income	99,019					98,946	73
Distributions paid to noncontrolling interests	(402)						(402)
Cash dividends	(19,735)				(19,735)	(19,735)
Stock-based compensation	12,964		12,964			12,964
Stock options exercised	15,823	$ 9	15,814			15,823
Stock options exercised (in shares)		920
Shares issued for Employee Stock Purchase Plan	4,707	$ 2	4,705			4,707
Shares issued for Employee Stock Purchase Plan (in shares)		209
Stock repurchases	(99,500)	$ (35)	(99,465)			(99,500)
Stock repurchases (in shares)		(3,468)
Tax benefit for stock options	(271)		(271)			(271)
Ending balance at Oct. 02, 2016	869,403	$ 570	260,340	(128,008)	736,357	869,259	144
Ending balance (in shares) at Oct. 02, 2016		57,042
Comprehensive income, net of tax:
Net income	117,917				117,874	117,874	43
Foreign currency translation adjustments	27,902			27,894		27,894	8
Gain on cash flow hedge valuations	1,614			1,614		1,614
Comprehensive income	147,433					147,382	51
Distributions paid to noncontrolling interests	(24)						(24)
Cash dividends	(21,672)				(21,672)	(21,672)
Stock-based compensation	13,450		13,450			13,450
Stock options exercised	15,094	$ 10	15,084			15,094
Stock options exercised (in shares)		907
Shares issued for Employee Stock Purchase Plan	4,940	$ 2	4,938			4,940
Shares issued for Employee Stock Purchase Plan (in shares)		190
Stock repurchases	(100,000)	$ (23)	(99,977)			(100,000)
Stock repurchases (in shares)		(2,266)
Ending balance at Oct. 01, 2017	928,624	$ 559	193,835	(98,500)	832,559	928,453	171
Ending balance (in shares) at Oct. 01, 2017		55,873
Comprehensive income, net of tax:
Net income	136,957				136,883	136,883	74
Foreign currency translation adjustments	(29,720)			(29,656)		(29,656)	(64)
Gain on cash flow hedge valuations	806			806		806
Comprehensive income	108,043					108,033	10
Distributions paid to noncontrolling interests	(52)						(52)
Cash dividends	(24,477)				(24,477)	(24,477)
Stock-based compensation	19,582		19,582			19,582
Restricted & performance shares released	(8,871)	$ 3	(8,874)			(8,871)
Restricted & performance shares released (in shares)		277
Stock options exercised	$ 13,511	$ 5	13,506			13,511
Stock options exercised (in shares)	549	549
Shares issued for Employee Stock Purchase Plan	$ 5,740	$ 1	5,739			5,740
Shares issued for Employee Stock Purchase Plan (in shares)		142
Stock repurchases	(75,000)	$ (15)	(74,985)			(75,000)
Stock repurchases (in shares)		(1,492)
Ending balance at Sep. 30, 2018	$ 967,100	$ 553	$ 148,803	$ (127,350)	$ 944,965	$ 966,971	$ 129
Ending balance (in shares) at Sep. 30, 2018		55,349