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Unrecognized cost of unvested share-based compensation awards.

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Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Feb. 21, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 18, 2019	Dec. 31, 2015
Scheduled to vest unvested shares
Share-based compensation expense			$ 2,224	$ 2,155	$ 2,195
Distribution to common shareholders (in dollars per share)			$ 1.56	$ 1.56	$ 1.56
Common stock dividends, ordinary income distribution percentage			38.13%	88.44%	58.77%
Common stock dividends, capital distribution percentage			0.00%	0.00%	40.67%
Common stock dividends, qualified distribution percentage			1.16%	0.71%	0.56%
Common stock dividends, capital gain distribution percentage			61.87%	10.85%	0.00%
Certain Our Officers And Employees Of RMR LLC
Scheduled to vest unvested shares
Shares purchased from certain of our officers and other employees of RMR LLC (in shares)			22,999	17,170	19,230
Share Award Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining common shares available for issuance (in shares)			2,382,195
Number of unvested shares (in shares)			161,990	148,010	153,410		158,874
Scheduled to vest unvested shares
2019 (in shares)			69,490
2020 (in shares)			42,110
2021 (in shares)			31,330
2022 (in shares)			19,060
Estimated future compensation for the unvested shares			$ 2,267
Weighted average period over which the compensation expense will be recorded			2 years
Share Award Plans \| Trustees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares awarded (in shares)		3,000
Aggregate market value of shares awarded		$ 47
Share Award Plans \| RMR LLC \| Officers and Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares awarded (in shares)			105,800	88,100	79,650
Aggregate market value of shares awarded			$ 2,022	$ 1,743	$ 1,724
Award vesting period			5 years
Share Award Plans \| RMR LLC \| Trustees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares awarded (in shares)			3,000	3,000	2,500
Aggregate market value of shares awarded			$ 248	$ 319	$ 228
Market value of shares awarded to each Trustee			$ 50	$ 64	$ 46
Subsequent Event
Scheduled to vest unvested shares
Distribution to common shareholders (in dollars per share)	$ 0.39
Distribution declared to common shareholders (in dollars per share)						$ 0.39
Distribution declared to common shareholders						$ 92,715
Dividends	$ 92,715