Dispositions of Real Estate and Discontinued Operations - Assets and Liabilities for Discontinued Operations (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) property	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) property loan	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) property	Dec. 31, 2019 USD ($)
Real estate:
Cash and cash equivalents	$ 14,005	$ 57,322	$ 14,005	$ 57,322	$ 53,085	$ 63,834
Restricted cash	3	19,263	3	19,263	17,168	$ 27,040
Intangible assets, net					25,000
Total assets of discontinued operations, net	105,009		105,009		2,626,306
LIABILITIES
Total liabilities of discontinued operations, net	18,910		18,910		415,737
Allowance for doubtful accounts receivable	4,583		4,583		5,873
Goodwill	0		0
Right-of-use asset	218,524		218,524		192,349
Financing receivable, after allowance for credit loss	411,062		411,062		195,375
Deferred revenue	782,413		782,413		774,316
Mortgage Debt
LIABILITIES
Debt instrument, periodic payment	1,000	1,000	4,000	4,000
Held-for-sale
Real estate:
Buildings and improvements	98		98		2,553,254
Development costs and construction in progress	54		54		21,509
Land	0		0		355,803
Accumulated depreciation and amortization	0		0		(615,708)
Net real estate	152		152		2,314,858
Investments in and advances to unconsolidated joint ventures	0		0		5,842
Accounts receivable, net of allowance of $4,583 and $5,873	4,826		4,826		20,500
Cash and cash equivalents	14,005		14,005		53,085
Restricted cash	3		3		17,168
Intangible assets, net	0		0		24,541
Right-of-use asset, net	30		30		4,109
Other assets, net	3,243		3,243		103,965
Total assets of discontinued operations, net	22,259		22,259		2,544,068
Assets held for sale, net	82,750		82,750		82,238
Assets held for sale and discontinued operations, net	105,009		105,009		2,626,306
LIABILITIES
Mortgage debt(4)	0		0		318,876
Lease liability	30		30		3,189
Accounts payable, accrued liabilities, and other liabilities	18,442		18,442		79,411
Deferred revenue	84		84		11,442
Total liabilities of discontinued operations, net	18,556		18,556		412,918
Liabilities related to assets held for sale, net	354		354		2,819
Liabilities related to assets held for sale and discontinued operations, net	18,910		18,910		415,737
Goodwill	0		0		29,000
Net real estate assets	30,000		30,000		73,000
Right-of-use asset	600		$ 600
Number of loans receivable | loan			1
Financing receivable, after allowance for credit loss	$ 51,000		$ 51,000
Deferred revenue					$ 2,000
Held-for-sale | Senior housing triple-net
LIABILITIES
Number of properties classified as held for sale | property	0		0		41
Held-for-sale | SHOP
LIABILITIES
Number of properties classified as held for sale | property	0		0		97
Held-for-sale | Senior Housing Operating Portfolio Joint Venture
LIABILITIES
Number of properties classified as held for sale | property					1
Held-for-sale | Medical office
LIABILITIES
Number of properties classified as held for sale | property	5		5		6
Held-for-sale | Life science
LIABILITIES
Number of properties classified as held for sale | property	1		1
Discontinued Operations | Mortgage Debt
LIABILITIES
Debt instrument, periodic payment	$ 37,000	$ 1,000	$ 318,000	$ 7,000