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Summary of Significant Accounting Policies - Investments (Details) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2017 | Dec. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Investment in available for sale securities |  |  |  |  |
Unrealized gains (losses) on equity securities, net |  |  | $ (20,724) | $ 0 | $ 0
Impairment of assets |  |  | 66,346 | $ 5,082 | $ 18,674
Five Star |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investment in common shares (in shares) |  |  |  | 4,235,000 |
Impairment of assets |  | $ 2,795 |  |  |
Weighted average quoted market prices (in dollars per share) |  |  |  |  | $ 2.70
RMR Inc. |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investment, difference between carrying value and fair value |  |  | 3,772 | $ 3,772 | $ 3,772
Unamortized liability for excess of fair value to carrying value |  |  | 64,390 |  |
RMR Inc. |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Fair value of investment |  |  | 139,994 | 156,398 |
Unrealized gains (losses) on equity securities, net |  |  | $ 70,168 | $ 86,572 |
RMR Inc. | Class A common shares |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investment in common shares (in shares) |  |  | 2,637,408 | 2,637,408 |
Cost basis for shares owned |  |  | $ 69,826 | $ 69,826 |
Unrealized gains (losses) on equity securities, net |  |  | $ (16,404) |  |
Five Star |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investment in common shares (in shares) |  |  | 4,235,000 |  |
Fair value of investment |  |  | $ 2,033 | 6,353 |
Unrealized gains (losses) on equity securities, net |  |  | $ (4,320) | $ 0 |
Impairment of assets | $ 5,082 |  |  |  |
Five Star | Common Shares |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investment in common shares (in shares) |  |  | 4,235,000 |  |
Cost basis for shares owned |  |  | $ 6,353 |  |

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