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Fair Value of Assets and Liabilities - Assets and Liabilities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Unrealized losses on equity securities | $ (20,724) | $ 0 | $ 0
RMR Inc. |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Unrealized losses on equity securities | $ 70,168 | $ 86,572 |
RMR Inc. | Class A common shares |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investment in common shares (in shares) | 2,637,408 | 2,637,408 |
Cost basis for shares owned | $ 69,826 | $ 69,826 |
Unrealized losses on equity securities | $ (16,404) |  |
Five Star |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investment in common shares (in shares) | 4,235,000 |  |
Unrealized losses on equity securities | $ (4,320) | $ 0 |
Five Star | Common Shares |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investment in common shares (in shares) | 4,235,000 |  |
Cost basis for shares owned | $ 6,353 |  |
Fair Value, Measurements, Recurring | RMR Inc. |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments | 139,994 |  |
Fair Value, Measurements, Recurring | RMR Inc. | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments | 139,994 |  |
Fair Value, Measurements, Recurring | RMR Inc. | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments | 0 |  |
Fair Value, Measurements, Recurring | RMR Inc. | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments | 0 |  |
Fair Value, Measurements, Recurring | Five Star |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments | 2,033 |  |
Fair Value, Measurements, Recurring | Five Star | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments | 2,033 |  |
Fair Value, Measurements, Recurring | Five Star | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments | 0 |  |
Fair Value, Measurements, Recurring | Five Star | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments | $ 0 |  |

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