Earnings per Share - Tabular Disclosure (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Net income
Income from continuing operations	$ 27,007	$ 13,174
Loss from discontinued operations, net of tax	(979)	(117)
Net income	26,028	13,057
Net income attributable to Brooks Automation, Inc. - basic	26,028	13,057
Net income attributable to Brooks Automation, Inc. - diluted	$ 26,028	$ 13,057
Weighted average common shares outstanding used in computing diluted earnings per share
Weighted average common shares outstanding used in computing basic earnings per share (in shares)	74,021	72,972
Dilutive restricted stock units	262	673
Weighted average common shares outstanding used in computing diluted earnings per share (in shares)	74,283	73,645
Basic net income per share:
Income from continuing operations (in dollars per share)	$ 0.36	$ 0.18
Loss from discontinued operations, net of tax (in dollars per share)	(0.01)	0.00
Basic net income per share attributable to Brooks Automation, Inc. (in dollars per share)	0.35	0.18
Diluted net income per share:
Income from continuing operations (in dollars per share)	0.36	0.18
Loss from discontinued operations, net of tax (in dollars per share)	(0.01)	0.00
Diluted net income per share (in dollars per share)	0.35	0.18
Dividend declared per share (in dollars per share)	$ 0.10	$ 0.10