Fair Value of Assets and Liabilities - Assets and Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) property shares	Mar. 31, 2019 USD ($)	Jan. 01, 2020 USD ($) shares	Dec. 31, 2019 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common shares of beneficial interest, shares issued (in shares) | shares	237,893,725			237,897,163
Common shares of beneficial interest, shares outstanding (in shares) | shares	237,893,725			237,897,163
Impairment of assets	$ 11,234	$ 6,206
Gains and losses on equity securities, net	$ (9,943)	$ 22,932
Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common shares of beneficial interest, shares issued (in shares) | shares			10,268,158
Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities investment (in shares) | shares	10,691,658
Initial cost basis	$ 44,448		$ 38,095
Common shares of beneficial interest, shares outstanding (in shares) | shares				423,500
Gains and losses on equity securities, net	(9,943)
Recurring | Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in affiliates, fair value	29,723
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	29,723
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) | 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	98,927
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	98,927
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
Common Shares | Five Star
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities investment (in shares) | shares	10,691,658
Medical Office Buildings
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of real estate properties held for sale | property	10
Medical Office Buildings | Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Sale price less estimated cost to sell	$ 1,658
Medical Office Buildings | Held-for-sale, Not Discontinued Operations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of assets	6,505
Medical Office Buildings | Held-for-sale, Not Discontinued Operations | Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of assets	$ 6,544
Number of real estate properties held for sale | property	6
Sale price less estimated cost to sell	$ 51,640
Senior Living Communities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of real estate properties held for sale | property	14
Senior Living Communities | Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Sale price less estimated cost to sell	$ 49,600
Senior Living Communities | Held-for-sale, Not Discontinued Operations | Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of assets	$ 5,581
Number of real estate properties held for sale | property	5
Sale price less estimated cost to sell	$ 655