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 ($) employee	Mar. 27, 2016 USD ($) $ / shares	Mar. 29, 2015 CAD	Mar. 29, 2015 USD ($) $ / shares	Dec. 28, 2014 CAD	Dec. 28, 2014 USD ($)	Sep. 28, 2014 USD ($)	Dec. 29, 2013 CAD	Mar. 27, 2016 USD ($) $ / shares	Mar. 29, 2015 USD ($) $ / shares	Sep. 27, 2015 USD ($)
Business acquisition
Contingent earn-out liability		$ 10,800							$ 10,800
Aggregate maximum of contingent consideration		17,800							17,800
Increase (Decrease) in contingent consideration									2,800
Gain (loss) on change in contingent consideration						$ (1,000)
Contingent consideration liability											$ 0
Gains (loss) on fair value adjustment in operating income									(2,800)
Severance including change in control payments		7,280
Professional services		5,685
Real estate-related		2,946
Total		15,911							15,911
Payments for acquisition and integration costs									11,700
Pre-payment charges paid		1,900
Estimated fair values of the assets acquired and liabilities assumed
Goodwill		716,244							$ 716,244		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		104,454							104,454
Other assets		6,142							6,142
Current liabilities		(74,641)							(74,641)
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		647,612		$ 683,436					1,319,281	$ 1,376,599
Operating income		32,617		29,046					62,766	48,670
Net income attributable to Tetra Tech		$ 20,038		$ 15,844					$ 39,526	$ 22,978
Earnings per share attributable to Tetra Tech Basic (in dollars per share) | $ / shares		$ 0.34		$ 0.26					$ 0.67	$ 0.37
Earnings per share attributable to Tetra Tech Diluted (in dollars per share) | $ / shares		$ 0.34		$ 0.26					$ 0.67	$ 0.37
Revenue since date of acquisition		$ 88,200
Operating income since date of acquisition		300
Amortization of intangible assets since date of acquisition		1,600
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
Gains (loss) on fair value adjustment in operating income		$ (1,800)
Caber
Business acquisition
Contingent earn-out liability				$ 0						$ 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						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