Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 05, 2015	Dec. 31, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percent of gross collected rents				3.00%
Percent of construction supervision				5.00%
Property management fees revenue				$ 47,690	$ 45,550	$ 42,482
Equity based compensation expense and related reimbursements				4,912	6,461	7,421
Due from related parties				90,369	102,759
Other assets, net of amortization				$ 143,727	153,143
Managed Equity REITs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of historical cost of transferred real estate assets				0.50%
Percentage of average invested capital below threshold limit				0.70%
Threshold amount, maximum				$ 250,000
Percentage of average invested capital above threshold limit				0.50%
Threshold limit, minimum				$ 250,000
Percentage of average market capitalization below threshold limit				0.70%
Percentage of average market capitalization above threshold limit				0.50%
Remaining term after termination (in years)				10 years
Aggregate annual base business management fees				$ 94,354	103,800	120,448
Incentive management fee percentage				12.00%
Aggregate incentive business management fees				$ 0	120,094	155,881
Due from related parties				$ 85,459	94,301
Cash and shares received from related party	$ 167,764
Other assets, net of amortization	$ 193,806
Managed Operators, ABP Trust and AIC
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of average invested capital above threshold limit				0.50%
Aggregate annual business management fees				$ 25,906	28,471	27,609
Incentive fee percentage condition 1		0.60%
AIC
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percent of total premiums paid			3.00%
Due from related parties				$ 7	7
Open End Fund
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percent of net asset value				1.00%
Due from related parties				$ 0	4,567
Up C Transaction | Managed Equity REITs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization of other assets				$ 9,416	9,416	9,416
RMRM
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Advisory fee percentage based on daily managed assets				0.85%
RMR LLC | Up C Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Duration of written notice for convenience (in days)				60 days
Window after calendar year end for written notice (in days)				60 days
Duration of written notice after change of control (in months)				12 months
Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Incentive fee percentage condition 1				20.00%
Incentive fee percentage condition 2				7.00%
Incentive fee percentage condition 1				1.50%
Minimum | Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Advisory fee percentage based on aggregate principal amounts of loan originated				0.50%
Maximum | Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Advisory fee percentage based on aggregate principal amounts of loan originated				1.00%
Advisory services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues				$ 2,911	3,169	4,352
Advisory services | RMR Advisors and Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues				2,767	3,013	2,888
Advisory services | Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues				144	156	1,464
Management services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues				168,766	178,075	191,594
Management services | Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues				816	194	1,055
Reimbursable compensation and benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues				57,256	57,490	53,152
Other client company reimbursable expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues				360,572	354,540	$ 0
Other client company reimbursable expenses | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues				360,572	354,540
Due from related parties				$ 56,079	$ 65,909