Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests
Beginning balance (in shares) at Sep. 30, 2018		55,349,000
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 income (loss)	(20,629)			(20,867)		(20,867)	238
Distributions paid to noncontrolling interests	(276)						(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)		179,000
Restricted & performance shares released	(6,802)	$ 2	(6,804)			(6,802)
Stock options exercised (in shares)		151,000
Stock options exercised	3,834	$ 2	3,832			3,834
Shares issued for Employee Stock Purchase Plan (in shares)		148,000
Shares issued for Employee Stock Purchase Plan	6,843	$ 1	6,842			6,843
Stock repurchases (in shares)		(880,000)
Stock repurchases	(50,000)	$ (9)	(49,991)			(50,000)
Ending balance (in shares) at Mar. 31, 2019		54,947,000
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		55,326,000
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 income (loss)	6,477			6,476		6,476	1
Distributions paid to noncontrolling interests	(21)						(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)		5,000
Restricted & performance shares released	(66)	$ 0	(66)			(66)
Stock options exercised (in shares)		65,000
Stock options exercised	1,520	$ 1	1,519			1,520
Stock repurchases (in shares)		(449,000)
Stock repurchases	(25,000)	$ (5)	(24,995)			(25,000)
Ending balance (in shares) at Mar. 31, 2019		54,947,000
Ending balance at Mar. 31, 2019	990,605	$ 549	111,277	(148,217)	1,026,836	990,445	160
Beginning balance (in shares) at Sep. 29, 2019		54,565,000
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	83,730				83,707	83,707	23
Other comprehensive income (loss)	(38,809)			(38,807)		(38,807)	(2)
Distributions paid to noncontrolling interests	(69)						(69)
Cash dividends per common share	(16,414)				(16,414)	(16,414)
Stock-based compensation	9,437		9,437			9,437
Restricted & performance shares released (in shares)		215,000
Restricted & performance shares released	(11,098)	$ 2	(11,100)			(11,098)
Stock options exercised (in shares)		269,000
Stock options exercised	7,927	$ 3	7,924			7,927
Shares issued for Employee Stock Purchase Plan (in shares)		168,000
Shares issued for Employee Stock Purchase Plan	8,715	$ 1	8,714			8,715
Stock repurchases (in shares)		(1,075,000)
Stock repurchases	(82,645)	$ (11)	(82,634)			(82,645)
Ending balance (in shares) at Mar. 29, 2020		54,142,000
Ending balance at Mar. 29, 2020	950,238	$ 541	10,473	(199,391)	1,138,485	950,108	130
Beginning balance (in shares) at Dec. 29, 2019		54,728,000
Beginning balance at Dec. 29, 2019	1,026,765	$ 547	60,747	(145,015)	1,110,312	1,026,591	174
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36,413				36,397	36,397	16
Other comprehensive income (loss)	(54,380)			(54,376)		(54,376)	(4)
Distributions paid to noncontrolling interests	(56)						(56)
Cash dividends per common share	(8,224)				(8,224)	(8,224)
Stock-based compensation	4,955		4,955			4,955
Restricted & performance shares released (in shares)		30,000
Restricted & performance shares released	(280)	$ 1	(281)			(280)
Stock options exercised (in shares)		215,000
Stock options exercised	6,513	$ 2	6,511			6,513
Stock repurchases (in shares)		(831,000)
Stock repurchases	(61,468)	$ (9)	(61,459)			(61,468)
Ending balance (in shares) at Mar. 29, 2020		54,142,000
Ending balance at Mar. 29, 2020	$ 950,238	$ 541	$ 10,473	$ (199,391)	$ 1,138,485	$ 950,108	$ 130