Related Person Transactions - Tax Receivable Agreement (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Payments under tax receivable agreement	$ 2,111	$ 2,266	$ 2,962
ABP Trust
Related Party Transaction [Line Items]
Tax receivable agreement tax benefit			24,710
ABP Trust | Up C Transaction
Related Party Transaction [Line Items]
Tax receivable agreement, percent of payment	85.00%
Payments under tax receivable agreement	$ 2,111	2,266	2,962
Tax receivable agreement amount payable	29,950
Tax receivable agreement obligations, current	2,161
RMR LLC
Related Party Transaction [Line Items]
Distributions paid	16,606	41,099	47,940
RMR LLC | ABP Trust
Related Party Transaction [Line Items]
Distributions paid	14,939	37,975	44,490
RMR LLC | ABP Trust and Managed REIT
Related Party Transaction [Line Items]
Distributions paid	$ 31,545	$ 79,074	$ 92,430