Mergers and Acquisitions (Details) $ / shares in Units, $ in Thousands, CAD in Millions		3 Months Ended												6 Months Ended	12 Months Ended
	Jan. 18, 2016 USD ($) employee	Oct. 02, 2016 USD ($)	Jun. 26, 2016 USD ($)	Mar. 27, 2016 USD ($)	Mar. 29, 2015 CAD	Mar. 29, 2015 USD ($)	Sep. 28, 2014 USD ($)	Jun. 29, 2014 USD ($)	Mar. 30, 2014 USD ($)	Dec. 29, 2013 CAD	Dec. 29, 2013 USD ($)	Mar. 31, 2013 CAD	Mar. 31, 2013 USD ($)	Mar. 30, 2014 USD ($)	Oct. 02, 2016 USD ($) $ / shares	Sep. 27, 2015 USD ($) $ / shares	Sep. 28, 2014 USD ($)	Oct. 02, 2016 USD ($)	Sep. 27, 2015 USD ($)	Sep. 28, 2014 USD ($)	Dec. 29, 2013 USD ($)
Business acquisition
Contingent earn-out liability																		$ 8,800	$ 0
Aggregate maximum of contingent consideration																		15,500
Increase (Decrease) in contingent consideration															$ 2,800
Gain (loss) on change in contingent consideration															(2,800)	$ (1,000)
Estimated fair values of the assets acquired and liabilities assumed
Goodwill																		717,988	601,379	$ 714,190
Pro forma operating results
Severance including change in control payments															10,917
Professional services															5,685
Real estate-related															2,946
Total															19,548
Pre-payment charges paid				$ 1,900											1,900
Contingent consideration liability		$ 8,757					$ 7,030				$ 81,789			$ 81,789	4,169	7,030	$ 81,789	8,757	4,169	7,030
Estimated contingent earn-out liabilities
Beginning balance (at fair value)											$ 81,789			81,789	4,169	7,030	81,789
Estimated earn-out liabilities for acquisition during the fiscal year															4,745	4,100	6,242
Increases due to re-measurement of fair value reported in interest expense															271	136	1,846
Net increase (decrease) due to re-measurement of fair value reported as losses (gains) in operating income															2,823	(3,113)	(58,694)
Foreign exchange impact																(785)	(3,507)
Earn-out payments
Reported as cash used in operating activities																	(1,984)
Reported as cash used in financing activities															(3,251)	(3,199)	(18,662)
Ending balance (at fair value)		8,757					7,030								$ 8,757	4,169	7,030
Maximum
Pro forma operating results
Period for contingent earn-out payments															3 years
Minimum
Pro forma operating results
Period for contingent earn-out payments															2 years
CEG
Business acquisition
Aggregate fair value of purchase prices																15,900
Cash paid to the sellers																11,800
Contingent earn-out liability																			4,100
Aggregate maximum of contingent consideration																			$ 9,800
Increase (Decrease) in contingent consideration															$ 1,800
Gain (loss) on change in contingent consideration															(1,800)
Coffey
Business acquisition
Aggregate fair value of purchase prices	$ 76,100
Number of employees | employee	3,300
Secured bank debt assumed	$ 37,100
Unsecured corporate bonds assumed	28,000
Estimated fair values of the assets acquired and liabilities assumed
Borrowings	$ (65,100)
Pro forma operating results
Total		$ 2,600	$ 1,000	$ 15,900											19,500
INDUS
Business acquisition
Aggregate fair value of purchase prices			18,700
Cash paid to the sellers			14,000
Contingent earn-out liability			4,700
Aggregate maximum of contingent consideration			$ 8,000
Earn-out period for operating income projection			2 years
Coffey and INDUS
Estimated fair values of the assets acquired and liabilities assumed
Accounts receivable																		71,515
Other current assets																		18,869
Property and equipment																		14,218
Goodwill																		107,618
Backlog and trade name intangible assets																		29,445
Other assets																		747
Current liabilities																		(77,606)
Borrowings																		(65,086)
Other long-term liabilities																		(4,885)
Net assets acquired																		$ 94,835
Pro forma operating results
Revenue															2,714,658	2,749,653
Operating income															152,676	73,209
Net income attributable to Tetra Tech															$ 98,871	$ 20,689
Earnings per share attributable to Tetra Tech Basic (in dollars per share) | $ / shares															$ 1.70	$ 0.33
Earnings per share attributable to Tetra Tech Diluted (in dollars per share) | $ / shares															$ 1.68	$ 0.33
Revenue since date of acquisition															$ 320,600
Operating income since date of acquisition															13,600
Amortization of intangible assets since date of acquisition															$ 6,700
Coffey and INDUS | Maximum | Backlog And Trade Name Member
Estimated fair values of the assets acquired and liabilities assumed
Useful life of intangible assets															5 years
Coffey and INDUS | Maximum | Trade names
Estimated fair values of the assets acquired and liabilities assumed
Useful life of intangible assets															5 years
Coffey and INDUS | Minimum | Backlog And Trade Name Member
Estimated fair values of the assets acquired and liabilities assumed
Useful life of intangible assets															1 year
Coffey and INDUS | Minimum | Trade names
Estimated fair values of the assets acquired and liabilities assumed
Useful life of intangible assets															3 years
Coffey and INDUS | Weighted average | Backlog And Trade Name Member
Estimated fair values of the assets acquired and liabilities assumed
Useful life of intangible assets															3 years
Caber
Business acquisition
Contingent earn-out liability						$ 0
Aggregate maximum of contingent consideration | CAD										CAD 8.0
Increase (Decrease) in contingent consideration							1,000									$ (3,100)
Gain (loss) on change in contingent consideration							(1,000)									$ 3,100
Earn-out period for operating income projection										2 years	2 years
Pro forma operating results
Potential earn-out to be paid each year | CAD										CAD 4.0
Minimum operating income threshold to earn contingent consideration during year one | CAD										4.0
Minimum operating income threshold to earn contingent consideration during year two | CAD										4.6
Operating income to earn maximum consideration during the first year | CAD										4.4
Operating income to earn maximum consideration during the second year | CAD										5.1
Initial fair value of the contingent consideration | CAD										CAD 6.5
Estimated potential earn-out (as a percent)										81.00%	81.00%
Potential maximum earn-out to be paid							4,000
Earn-out paid to former owners					CAD 4.0	3,200
Gains (loss) on fair value adjustment in operating income						$ 3,100
Parkland
Business acquisition
Contingent earn-out liability									$ 0					0						0	$ 0
Gain (loss) on change in contingent consideration							19,900		19,100		$ 5,600			$ 24,700
Earn-out period for operating income projection												3 years	3 years
Pro forma operating results
Minimum operating income threshold to earn contingent consideration during year one | CAD												CAD 34.7
Minimum operating income threshold to earn contingent consideration during year two | CAD												38.2
Minimum operating income threshold to earn contingent consideration during year three | CAD												41.9
Operating income to earn maximum consideration during the first year | CAD												42.5
Operating income to earn maximum consideration during the second year | CAD												46.4
Operating income to earn maximum consideration during the third year | CAD												50.6
Initial fair value of the contingent consideration | CAD												CAD 46.8
Estimated potential earn-out (as a percent)												84.00%	84.00%
Potential maximum earn-out to be paid | CAD												CAD 56.0
Potential earn-out to be paid in the first year | CAD												12.0
Potential earn-out to be paid in the second year | CAD												22.0
Potential earn-out to be paid in the third year | CAD												CAD 22.0
Net gains on fair value adjustments in operating income																	44,600
Net decrease in contingent earn-out liabilities																	44,600
Profit previously recognized on project									5,300
AEG
Business acquisition
Contingent earn-out liability																				$ 0
Increase (Decrease) in contingent consideration								$ (8,900)			1,000
Gain (loss) on change in contingent consideration								$ 8,900			$ (1,000)
Earn-out period for operating income projection												2 years	2 years
Pro forma operating results
Minimum operating income threshold to earn contingent consideration during year one													$ 10,000
Minimum operating income threshold to earn contingent consideration during year two													11,000
Operating income to earn maximum consideration during the first year													17,500
Operating income to earn maximum consideration during the second year													18,500
Initial fair value of the contingent consideration													$ 21,500
Estimated potential earn-out (as a percent)												79.00%	79.00%
Potential maximum earn-out to be paid													$ 27,100
Net gains on fair value adjustments in operating income							4,500										$ 13,200
Annual payment amount													11,300
One-time payment													4,500
Minimum operating income in both years for earning one-time payment							9,000						9,000
Earn-out provision paid for meeting second earn out provision							$ 4,500						$ 4,500
Earn-outs paid to former owners reported as cash used in financing activities									$ 9,100