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Related Person Transactions (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	Feb. 13, 2020 USD ($)	Jul. 01, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) party agreement	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2020 shares
Related Party Transaction [Line Items]
Proceeds from sale of RMR Inc. common shares, net			$ 98,557	$ 0	$ 0
Equity method investments, carrying value			1,571	142,027
Equity in earnings of an investee			400	516	608
Equity in unrealized gain (loss) of an investee			$ 91	(68)	462
The RMR Group Inc
Related Party Transaction [Line Items]
Number of unrelated parties \| party			4
Office Portfolio
Related Party Transaction [Line Items]
Standalone insurance policy coverage period			1 year
Five Star
Related Party Transaction [Line Items]
Minimum percentage of ownership of lessee's voting stock above which the entity has the option to cancel all its rights			9.80%
RMR Inc. \| Class A common shares
Related Party Transaction [Line Items]
Investment in common shares (in shares) \| shares		2,637,408
Sale of stock, price per share (in dollars per share) \| $ / shares		$ 40.00
Proceeds from sale of RMR Inc. common shares, net		$ 98,557
Five Star
Related Party Transaction [Line Items]
Maximum percentage of any class of equity shares that can be acquired without approval			9.80%
Five Star \| Subsequent Event
Related Party Transaction [Line Items]
Investment in common shares (in shares) \| shares						10,691,658
Percentage of total shares outstanding						33.90%
RMR LLC
Related Party Transaction [Line Items]
Number of management agreements with related party \| agreement			2
Premium payable for combined directors' and officers', percentage			3.00%
Premium payable for combined directors' and officers' liability insurance policy			$ 167	253	255
RMR LLC \| RMR Inc. \| Year one
Related Party Transaction [Line Items]
Award vesting rights, percentage			20.00%
RMR LLC \| RMR Inc. \| Year two
Related Party Transaction [Line Items]
Award vesting rights, percentage			20.00%
RMR LLC \| RMR Inc. \| Year three
Related Party Transaction [Line Items]
Award vesting rights, percentage			20.00%
RMR LLC \| RMR Inc. \| Year four
Related Party Transaction [Line Items]
Award vesting rights, percentage			20.00%
RMR LLC \| RMR Inc. \| Year five
Related Party Transaction [Line Items]
Award vesting rights, percentage			20.00%
AIC
Related Party Transaction [Line Items]
Total premiums including taxes and fees			$ 4,413	2,433	3,607
Equity method investments, carrying value			298	8,632	8,185
AIC \| Subsequent Event
Related Party Transaction [Line Items]
Liquidation distribution	$ 9,000
RMR LLC
Related Party Transaction [Line Items]
Rental revenue from related party			$ 256	$ 228	$ 331
RMR LLC \| Five Star \| Subsequent Event
Related Party Transaction [Line Items]
Percentage of total shares outstanding						6.30%