Earnings per Share - Tabular Disclosure (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Income from continuing operations	$ 13,174	$ 6,266
(Loss) income from discontinued operations, net of tax	(117)	8,149
Net income	$ 13,057	$ 14,415
Weighted average common shares outstanding used in computing basic earnings per share (in shares)	72,972	71,450
Dilutive restricted stock units	673	715
Weighted average common shares outstanding used in computing diluted earnings per share (in shares)	73,645	72,165
Basic net income per share attributable to Brooks Automation, Inc. common stockholders:
Income from continuing operations	$ 0.18	$ 0.09
(Loss) income from discontinued operations, net of tax	0.00	0.11
Basic net income per share attributable to Brooks Automation, Inc. (in dollars per share)	0.18	0.20
Diluted net income per share:
Income from continuing operations	0.18	0.09
(Loss) income from discontinued operations, net of tax	0.00	0.11
Diluted net income per share attributable to Brooks Automation, Inc. common stockholders (in dollars per share)	0.18	0.20
Dividend declared per share (in dollars per share)	$ 0.10	$ 0.10