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Summary of Significant Accounting Policies - Investments (Details) $ in Thousands |  | 12 Months Ended |
--- | --- | --- | ---
Jan. 01, 2020 USD ($) shares | Dec. 31, 2020 USD ($) shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2018 USD ($) | Jun. 30, 2020 shares
Investment in available for sale securities |  |  |  |  |
Unrealized gains and losses on equity securities held |  | $ 34,106 | $ (462) |  |
Class A common shares | RMR Inc. |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investment in common shares (in shares) | shares |  |  |  |  | 2,637,408
Five Star |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Shares issued, value |  | $ 59,801 | 0 | $ 0 |
Five Star | Common Shares |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Issued (in shares) | shares | 10,268,158 |  |  |  |
Five Star |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investment in common shares (in shares) | shares |  | 10,691,658 |  |  |
Unrealized gains and losses on equity securities held |  |  | 462 |  |
Five Star | Recurring |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investments in Five Star |  | $ 73,772 | $ 1,571 |  |
Five Star | Common Shares |  |  |  |  |
Investment in available for sale securities |  |  |  |  |
Investment in common shares (in shares) | shares |  | 10,691,658 | 423,500 |  |
Reverse stock split ratio |  | 0.100 |  |  |
Shares issued, value | $ 38,095 |  |  |  |
Cost basis for shares owned |  | $ 44,448 |  |  |

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