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Fair Value of Assets and Liabilities - Assets and Liabilities Recurring and Nonrecurring Measured at Fair Value (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Assets and liabilities measured at fair value |  |  |  |
Investments in available for sale securities |  | $ 162,751 | $ 115,612 |
Impairment of assets |  | 5,082 | 18,674 | $ 194
Recurring |  |  |  |
Assets and liabilities measured at fair value |  |  |  |
Investments in available for sale securities |  | 162,751 |  |
Recurring | Level 1 |  |  |  |
Assets and liabilities measured at fair value |  |  |  |
Investments in available for sale securities |  | 162,751 |  |
Recurring | Level 2 |  |  |  |
Assets and liabilities measured at fair value |  |  |  |
Investments in available for sale securities |  | 0 |  |
Recurring | Level 3 |  |  |  |
Assets and liabilities measured at fair value |  |  |  |
Investments in available for sale securities |  | $ 0 |  |
Five Star |  |  |  |
Assets and liabilities measured at fair value |  |  |  |
Investment in common shares |  | 4,235,000 |  |
Common Shares | Five Star |  |  |  |
Assets and liabilities measured at fair value |  |  |  |
Investment in common shares |  | 4,235,000 |  |
Impairment of assets | $ 5,082 |  |  |
Amount of investment acquired |  | $ 6,353 |  |
Class A common shares | RMR Inc. |  |  |  |
Assets and liabilities measured at fair value |  |  |  |
Investment in common shares |  | 2,637,408 |  |
Amount of investment acquired |  | $ 69,826 |  |
The RMR Group Inc |  |  |  |
Assets and liabilities measured at fair value |  |  |  |
Investment in common shares |  | 2,637,408 |  |
Amount of investment acquired |  | $ 69,826 | 69,826 |
Unrealized gain |  | $ 86,572 | $ 34,352 |

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