Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2017	Jan. 01, 2017
Income Statement [Abstract]
Revenue	$ 759,749	$ 668,851
Subcontractor costs	(214,902)	(179,300)
Other costs of revenue	(450,702)	(408,190)
Gross profit	94,145	81,361
Selling, general and administrative expenses	(45,556)	(41,506)
Income from operations	48,589	39,855
Interest expense	(3,160)	(2,908)
Income before income tax benefit (expense)	45,429	36,947
Income tax benefit (expense)	623	(10,358)
Net income	46,052	26,589
Net income attributable to noncontrolling interests	(18)	(27)
Net income attributable to Tetra Tech	$ 46,034	$ 26,562
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.82	$ 0.47
Diluted (in dollars per share)	$ 0.81	$ 0.46
Weighted-average common shares outstanding:
Basic (in shares)	55,855	57,099
Diluted (in shares)	56,875	58,145
Cash dividends paid per share (in dollars per share)	$ 0.1	$ 0.09