Earnings per Share - Tabular Disclosure (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Net income attributable to Brooks Automation, Inc.
Income from continuing operations	$ 65,035	$ 9,554	$ 67,717
Income (loss) from discontinued operations, net of tax	(182)	427,862	48,747
Net income	64,853	437,416	116,464
Net loss attributable to noncontrolling interest			111
Net income attributable to Brooks Automation, Inc.	64,853	437,416	116,575
Net income attributable to Brooks Automation, Inc. - basic	64,853	437,416	116,575
Net income attributable to Brooks Automation, Inc. - diluted	$ 64,853	$ 437,416	$ 116,575
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)	73,557	71,992	70,489
Dilutive restricted stock units	293	394	448
Weighted average common shares outstanding used in computing diluted earnings per share (in shares)	73,850	72,386	70,937
Basic net income per share attributable to Brooks Automation, Inc. common stockholders:
Income from continuing operations (in dollars per share)	$ 0.88	$ 0.13	$ 0.96
Income (loss) from discontinued operations, net of tax (in dollars per share)	0.00	5.95	0.69
Basic net income per share attributable to Brooks Automation, Inc. (in dollars per share)	0.88	6.08	1.65
Diluted net income per share attributable to Brooks Automation, Inc. common stockholders:
Income from continuing operations (in dollars per share)	0.88	0.13	0.95
Income (loss) from discontinued operations, net of tax (in dollars per share)	0.00	5.91	0.69
Diluted net income per share attributable to Brooks Automation, Inc. common stockholders (in dollars per share)	0.88	6.04	1.64
Dividend declared per share (in dollars per share)	$ 0.40	$ 0.40	$ 0.40