Fair Value of Assets and Liabilities - Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain on equity securities	$ 22,932	$ 27,241
Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities investment (in shares)	4,235,000
Recurring | RMR Inc
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	$ 160,830
Recurring | Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	4,129
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | RMR Inc
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	160,830
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	4,129
Recurring | Significant Other Observable Inputs (Level 2) | RMR Inc
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	0
Recurring | Significant Other Observable Inputs (Level 2) | Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	0
Recurring | Significant Unobservable Inputs (Level 3) | RMR Inc
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	0
Recurring | Significant Unobservable Inputs (Level 3) | Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	0
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties held for sale	7,489
Nonrecurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties held for sale	0
Nonrecurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties held for sale	7,489
Nonrecurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties held for sale	$ 0
Class A common shares | RMR Inc
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities investment (in shares)	2,637,408
Historical cost basis for shares owned	$ 69,826
Unrealized gain on equity securities	$ 20,836
Common Shares | Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities investment (in shares)	4,235,000
Historical cost basis for shares owned	$ 6,353
Unrealized gain on equity securities	$ 2,096