Reconciliation of Weighted Average Shares Outstanding for Calculating Basic and Diluted Earnings (Loss) Per Share (Detail) (USD $) In Thousands, except Per Share data	12 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Earnings Per Share Disclosure [Line Items]
Net income (loss) attributable to Brooks Automation, Inc.	$ 128,352	$ 58,982	$ (227,858)
Weighted average common shares outstanding used in computing basic earnings (loss) per share	64,549	63,777	62,911
Dilutive common stock options and restricted stock awards	454	397
Weighted average common shares outstanding for purposes of computing diluted earnings (loss) per share	65,003	64,174	62,911
Basic net income (loss) per share attributable to Brooks Automation, Inc. common stockholders	$ 1.99	$ 0.92	$ (3.62)
Diluted net income (loss) per share attributable to Brooks Automation, Inc. common stockholders	$ 1.97	$ 0.92	$ (3.62)

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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+ Details

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References+ Details

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

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-Glossary Other Comprehensive Income

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-Glossary Net Income

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+ Details

- Definition

The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

+ References+ Details

- Definition

Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

+ References

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