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Related Person Transactions - Investment in RMR Inc (Details) $ in Thousands |  |  | 1 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Sep. 14, 2017 shares | May 18, 2017 shares | Sep. 30, 2017 USD ($) shares | Sep. 30, 2016 shares | Sep. 30, 2017 USD ($) building shares | Jun. 30, 2018 USD ($) | Dec. 31, 2016 USD ($)
Related person transactions |  |  |  |  |  |  |
Common shares granted (in shares) | 88,100 | 3,000 | 88,100 | 79,650 |  |  |
RMR Inc | Class A common shares |  |  |  |  |  |  |
Related person transactions |  |  |  |  |  |  |
Number of shares purchased (in shares) |  |  |  |  | 2,637,408 |  |
RMR LLC |  |  |  |  |  |  |
Related person transactions |  |  |  |  |  |  |
Shares purchased to satisfy tax withholding and payment obligations (in shares) |  |  | 16,654 | 17,667 |  |  |
AIC |  |  |  |  |  |  |
Related person transactions |  |  |  |  |  |  |
Property insurance term |  |  |  |  | 1 year |  |
Number of buildings covered by property insurance | building |  |  |  |  | 2 |  |
Equity method investments | $ |  |  | $ 7,945 |  | $ 7,945 |  | $ 7,116
Scenario, Forecast | AIC |  |  |  |  |  |  |
Related person transactions |  |  |  |  |  |  |
Equity method investments, property insurance annual premium amount | $ |  |  |  |  |  | $ 2,433 |

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