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Summary of Significant Accounting Policies - Investments (Details) - USD ($) $ / shares in Units, $ in Thousands |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Dec. 31, 2016 | Jun. 05, 2015 | Dec. 31, 2017 | Jun. 30, 2017 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment in available for sale securities |  |  |  |  |  |  |  |
Impairment of assets |  |  |  |  |  | $ 5,082 | $ 18,674 | $ 194
The RMR Group Inc |  |  |  |  |  |  |  |
Investment in available for sale securities |  |  |  |  |  |  |  |
Investment in common shares |  |  | 2,637,408 |  |  | 2,637,408 |  |
Amount of investment acquired | $ 69,826 |  | $ 69,826 |  | $ 69,826 | $ 69,826 | 69,826 |
Fair value of investment | 104,178 |  | 156,398 |  | 104,178 | 156,398 | 104,178 |
Unrealized gain | $ 34,352 |  | $ 86,572 |  | $ 34,352 | $ 86,572 | $ 34,352 |
The RMR Group Inc | Class A common shares |  |  |  |  |  |  |  |
Investment in available for sale securities |  |  |  |  |  |  |  |
Investment in common shares | 2,637,408 |  | 2,637,408 |  | 2,637,408 | 2,637,408 | 2,637,408 |
Five Star |  |  |  |  |  |  |  |
Investment in available for sale securities |  |  |  |  |  |  |  |
Investment in common shares | 4,235,000 |  | 4,235,000 |  | 4,235,000 | 4,235,000 | 4,235,000 |
Amount of investment acquired |  |  | $ 6,353 |  |  | $ 6,353 |  |
Impairment of assets |  |  |  |  | $ 2,795 |  |  |
Weighted average quoted market prices (in dollars per share) | $ 2.70 |  | $ 1.50 | $ 1.50 |  |  |  |
RMR Inc. | Up C Transaction |  |  |  |  |  |  |  |
Investment in available for sale securities |  |  |  |  |  |  |  |
Amount of investment acquired |  | $ 60,700 |  |  |  |  |  |
Fair value of investment |  | $ 136,278 |  |  |  |  |  |
Investment, difference between carrying value and fair value |  |  |  |  |  | 3,772 | $ 3,772 | $ 2,145
Unamortized liability for excess of fair value to carrying value |  |  | $ 68,161 |  |  | $ 68,161 |  |
Class A common shares | RMR Inc. | Up C Transaction |  |  |  |  |  |  |  |
Investment in available for sale securities |  |  |  |  |  |  |  |
Number of shares acquired (in shares) |  | 5,272,787 |  |  |  |  |  |

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