Leases - Assets and Liabilities - Operating Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Assets and Liabilities, Lessee [Abstract]
Operating lease right-of-use assets	$ 42,129	$ 39,071
Accrued expenses and other current liabilities	$ 7,937	7,015
Operating Lease Liability Current, Statement of Financial Position	brks:AccruedExpensesAndOtherCurrentLiabilities
Long-term operating lease liabilities	$ 34,173	31,855
Total lease liability balance	$ 42,110	$ 38,870