Quarterly Financial Information-Unaudited (Tables)

12 Months Ended

Sep. 30, 2018

Quarterly Financial Information Disclosure [Abstract]

Schedule of unaudited quarterly data

	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	(in thousands, except per share data)
Fiscal Year 2018
Revenue	$	759,749	$	700,262	$	764,795	$	739,343
Income from operations	48,589		42,716		55,496		43,285
Net income attributable to Tetra Tech	46,034		28,725		33,322		28,802
Earnings per share attributable to Tetra Tech:
Basic	$	0.82	$	0.51	$	0.60	$	0.52
Diluted	$	0.81	$	0.51	$	0.59	$	0.51
Weighted-average common shares outstanding:
Basic	55,855		55,841		55,537		55,341
Diluted	56,875		56,673		56,390		56,349
Fiscal Year 2017
Revenue	$	668,851	$	663,781	$	685,539	$	735,188
Income from operations	39,855		42,956		45,884		54,647
Net income attributable to Tetra Tech	26,562		26,862		29,983		34,467
Earnings per share attributable to Tetra Tech:
Basic	$	0.47	$	0.47	$	0.52	$	0.61
Diluted	$	0.46	$	0.46	$	0.52	$	0.60
Weighted-average common shares outstanding:
Basic	57,099		57,270		57,184		56,338
Diluted	58,145		58,270		58,161		57,326