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 Oct. 01, 2017		55,873
Beginning balance at Oct. 01, 2017	$ 928,624	$ 559	$ 193,835	$ (98,500)	$ 832,559	$ 928,453	$ 171
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	74,799				74,759	74,759	40
Other comprehensive (loss) income	(19,551)			(19,544)		(19,544)	(7)
Distributions paid to noncontrolling interests	(22)					0	(22)
Cash dividends per common share	(11,172)				(11,172)	(11,172)
Stock-based compensation	8,705		8,705			8,705
Restricted & performance shares released (in shares)		273
Restricted & performance shares released	(8,819)	$ 2	(8,821)			(8,819)
Stock options exercised (in shares)		481
Stock options exercised	11,938	$ 5	11,933			11,938
Shares issued for Employee Stock Purchase Plan (in shares)		142
Shares issued for Employee Stock Purchase Plan	5,726	$ 1	5,725			5,726
Stock repurchases (in shares)		(1,020)
Stock repurchases	(50,000)	$ (10)	(49,990)			(50,000)
Ending balance (in shares) at Apr. 01, 2018		55,749
Ending balance at Apr. 01, 2018	940,228	$ 557	161,387	(118,044)	896,146	940,046	182
Beginning balance (in shares) at Oct. 01, 2017		55,873
Beginning balance at Oct. 01, 2017	928,624	$ 559	193,835	(98,500)	832,559	928,453	171
Ending balance (in shares) at Sep. 30, 2018		55,349
Ending balance at Sep. 30, 2018	967,100	$ 553	148,803	(127,350)	944,965	966,971	129
Beginning balance (in shares) at Dec. 31, 2017		55,988
Beginning balance at Dec. 31, 2017	947,121	$ 560	175,293	(101,901)	873,004	946,956	165
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	28,747				28,725	28,725	22
Other comprehensive (loss) income	(16,148)			(16,143)		(16,143)	(5)
Cash dividends per common share	(5,583)				(5,583)	(5,583)
Stock-based compensation	4,735		4,735			4,735
Restricted & performance shares released (in shares)		2
Restricted & performance shares released	(7)	$ (1)	(6)			(7)
Stock options exercised (in shares)		264
Stock options exercised	6,363	$ 3	6,360			6,363
Stock repurchases (in shares)		(505)
Stock repurchases	(25,000)	$ (5)	(24,995)			(25,000)
Ending balance (in shares) at Apr. 01, 2018		55,749
Ending balance at Apr. 01, 2018	940,228	$ 557	161,387	(118,044)	896,146	940,046	182
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
Ending balance (in shares) at Dec. 30, 2018		55,326
Ending balance at Dec. 30, 2018	954,301	$ 553	130,753	(154,693)	977,541	954,154	147
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	97,976				97,907	97,907	69
Other comprehensive (loss) income	(20,629)			(20,867)		(20,867)	238
Distributions paid to noncontrolling interests	(276)					0	(276)
Cash dividends per common share	(13,270)				(13,270)	(13,270)
Stock-based compensation	8,595		8,595			8,595
Restricted & performance shares released (in shares)		179
Restricted & performance shares released	(6,802)	$ 2	(6,804)			(6,802)
Stock options exercised (in shares)		151
Stock options exercised	3,834	$ 2	3,832			3,834
Shares issued for Employee Stock Purchase Plan (in shares)		148
Shares issued for Employee Stock Purchase Plan	6,843	$ 1	6,842			6,843
Stock repurchases (in shares)		(880)
Stock repurchases	(50,000)	$ (9)	(49,991)			(50,000)
Ending balance (in shares) at Mar. 31, 2019		54,947
Ending balance at Mar. 31, 2019	990,605	$ 549	111,277	(148,217)	1,026,836	990,445	160
Beginning balance (in shares) at Dec. 30, 2018		55,326
Beginning balance at Dec. 30, 2018	954,301	$ 553	130,753	(154,693)	977,541	954,154	147
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	55,944				55,911	55,911	33
Other comprehensive (loss) income	6,477			6,476		6,476	1
Distributions paid to noncontrolling interests	(21)					0	(21)
Cash dividends per common share	(6,616)				(6,616)	(6,616)
Stock-based compensation	4,066		4,066			4,066
Restricted & performance shares released (in shares)		5
Restricted & performance shares released	(66)	$ 0	(66)			(66)
Stock options exercised (in shares)		65
Stock options exercised	1,520	$ 1	1,519			1,520
Stock repurchases (in shares)		(449)
Stock repurchases	(25,000)	$ (5)	(24,995)			(25,000)
Ending balance (in shares) at Mar. 31, 2019		54,947
Ending balance at Mar. 31, 2019	$ 990,605	$ 549	$ 111,277	$ (148,217)	$ 1,026,836	$ 990,445	$ 160