Postretirement Benefits - Determination of Pension Expense (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Compensation and Retirement Disclosure [Abstract]
Investment gains or losses recognition period	5 years
Cumulative unrecognized net actuarial gains (loss)	$ 1.5	$ 0.9
Cumulative unrecognized investment gains	$ 0.6	$ 0.5

Before tax net amount of cumulative unrecognized investment gain (loss) that are not yet recognized as a component of net periodic benefit cost, and that are recognized as increases (decreases) in other comprehensive income as they arise.

No definition available.

Name:	brks_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesCumulativeUnrecognizedInvestmentGainsLossesBeforeTax
Namespace Prefix:	brks_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration