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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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Amount of unrealized gain (loss) on investment in marketable security.

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