Mergers and Acquisitions (Details) $ / shares in Units, $ in Thousands, CAD in Millions		3 Months Ended								6 Months Ended	9 Months Ended		12 Months Ended
	Jan. 18, 2016 USD ($) employee	Jun. 26, 2016 USD ($) $ / shares	Mar. 27, 2016 USD ($)	Jun. 28, 2015 USD ($) $ / shares	Mar. 29, 2015 CAD	Mar. 29, 2015 USD ($)	Dec. 28, 2014 CAD	Sep. 28, 2014 USD ($)	Dec. 29, 2013 CAD	Mar. 27, 2016 USD ($)	Jun. 26, 2016 USD ($) $ / shares	Jun. 28, 2015 USD ($) $ / shares	Sep. 27, 2015 USD ($)
Business acquisition
Contingent earn-out liability		$ 11,800									$ 11,800
Aggregate maximum of contingent consideration		17,800									17,800
Increase (Decrease) in contingent consideration										$ 2,800	2,800
Gain (loss) on change in contingent consideration										(2,800)	(2,800)		$ (1,000)
Contingent consideration liability													0
Severance including change in control payments		1,005									8,285
Professional services											5,685
Real estate-related											2,946
Total		1,005									16,916
Pre-payment charges paid			$ 1,900
Estimated fair values of the assets acquired and liabilities assumed
Goodwill		727,773									$ 727,773		601,379
Maximum
Business acquisition
Earn-out period for operating income projection											3 years
Minimum
Business acquisition
Earn-out period for operating income projection											2 years
Coffey
Business acquisition
Number of employees | employee	3,300
Aggregate fair value of purchase prices	$ 76,100
Secured bank debt assumed	37,100
Unsecured corporate bonds assumed	$ 28,000
INDUS
Business acquisition
Aggregate fair value of purchase prices			17,800
Cash paid to the sellers			13,100
Contingent earn-out liability			4,700							4,700
Aggregate maximum of contingent consideration			$ 8,000							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		82,736									$ 82,736
Other current assets		7,846									7,846
Property and equipment		14,723									14,723
Goodwill		110,831									110,831
Other assets		6,142									6,142
Current liabilities		(81,018)									(81,018)
Borrowings		(65,086)									(65,086)
Other long-term liabilities		(7,945)									(7,945)
Noncontrolling interests		(500)									(500)
Net assets acquired		93,917									93,917
Pro forma operating results
Revenue		666,869		$ 691,637							1,986,149	$ 2,068,236
Operating income		40,090		23,803							102,855	71,469
Net income attributable to Tetra Tech		$ 26,397		$ 8,883							$ 65,923	$ 30,856
Earnings per share attributable to Tetra Tech Basic (in dollars per share) | $ / shares		$ 0.46		$ 0.15							$ 1.12	$ 0.50
Earnings per share attributable to Tetra Tech Diluted (in dollars per share) | $ / shares		$ 0.45		$ 0.15							$ 1.11	$ 0.49
Revenue since date of acquisition		$ 116,900									$ 205,100
Operating income since date of acquisition		6,400									8,300
Amortization of intangible assets since date of acquisition		2,100									3,700
Coffey and INDUS | Backlog and customer relationship
Estimated fair values of the assets acquired and liabilities assumed
Backlog and customer relationship intangible assets		$ 26,188									$ 26,188
Coffey and INDUS | Maximum | Backlog and customer relationship
Business acquisition
Useful life of intangible assets											5 years
Coffey and INDUS | Minimum | Backlog and customer relationship
Business acquisition
Useful life of intangible assets											1 year
Coffey and INDUS | Weighted average | Backlog and customer relationship
Business acquisition
Useful life of intangible assets											3 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	$ 1,800
Gain (loss) on change in contingent consideration										$ (1,800)	$ (1,800)
Caber
Business acquisition
Contingent earn-out liability						$ 0
Aggregate maximum of contingent consideration | CAD									CAD 8.0
Earn-out period for operating income projection									2 years
Increase (Decrease) in contingent consideration								$ 1,000
Gain (loss) on change in contingent consideration						3,100
Gains (loss) on fair value adjustment in operating income												$ 3,100
Potential earn-out to be paid each year | CAD									CAD 4.0
Potential maximum earn-out to be paid								4,000
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							CAD 4.6		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%
Earn-out paid to former owners					CAD 4.0	3,200
Estimated contingent earn-out liabilities
Decreases (increase) in contingent earn-out liabilities						$ 3,100		$ (1,000)				$ 3,100