Earnings per Share - Tabular Disclosure (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Income from continuing operations	$ 6,266	$ 1,619
Income from discontinued operations, net of tax	8,149	14,867
Net income	$ 14,415	$ 16,486
Weighted average common shares outstanding used in computing basic earnings per share (in shares)	71,450	70,183
Dilutive restricted stock units	715	681
Weighted average common shares outstanding used in computing diluted earnings per share (in shares)	72,165	70,864
Basic net income per share attributable to Brooks Automation, Inc. common stockholders:
Income from continuing operations (in dollars per share)	$ 0.09	$ 0.02
Income from discontinued operations, net of tax (in dollars per share)	0.11	0.21
Basic net income per share attributable to Brooks Automation, Inc. (in dollars per share)	0.20	0.23
Diluted net income per share attributable to Brooks Automation, Inc. common stockholders:
Income from continuing operations (in dollars per share)	0.09	0.02
Income from discontinued operations, net of tax (in dollars per share)	0.11	0.21
Diluted net income per share attributable to Brooks Automation, Inc. common stockholders (in dollars per share)	0.20	0.23
Dividend declared per share (in dollars per share)	$ 0.10	$ 0.10