Dispositions of Real Estate and Discontinued Operations - Dispositions of Real Estate (Details)		1 Months Ended								3 Months Ended			9 Months Ended		12 Months Ended
	Jan. 31, 2020 USD ($) property	Sep. 30, 2021 loan	Jun. 30, 2021 USD ($) property	May 31, 2021 USD ($) property	Apr. 30, 2021 USD ($) property	Mar. 31, 2021 USD ($) property	Feb. 28, 2021 USD ($) property	Jan. 31, 2021 USD ($) property	Dec. 31, 2020 USD ($) property	Sep. 30, 2021 USD ($) property	Mar. 31, 2021 USD ($) property	Sep. 30, 2020 USD ($) property	Sep. 30, 2021 USD ($) property	Sep. 30, 2020 USD ($) property	Dec. 31, 2020 USD ($) property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sales of real estate, net										$ 14,635,000		$ 2,283,000	$ 189,873,000	$ 85,636,000
Repayments of secured debt									$ 6,000,000
Number of properties disposed | property														18
Carrying value of mortgages assumed by buyer in real estate dispositions													$ 143,676,000	$ 0
Number of loans sold		1											5		3
Mortgage Debt
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets classified as discontinued operations | property				6	2	2			1		2				1
Repayments of secured debt					$ 64,000,000	$ 39,000,000
Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Abandonment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sales of real estate, net										42,000,000		2,000,000	$ 52,000,000	248,000,000	$ 283,000,000
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sales of real estate, net										$ 27,000,000		(2,000,000)	34,000,000	162,000,000	$ 193,000,000
Brookedale MTCA
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sales of real estate, net	$ 164,000,000
Number of properties disposed | property	18														18
Sunrise Senior Housing Portfolio | SHOP
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Capital expenditure funding, amount committed			$ 56,000,000					$ 92,000,000
Capital expenditure funding, amount funded													400,000
Oakmont SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Carrying value of mortgages assumed by buyer in real estate dispositions					$ 107,000,000
Oakmont SHOP Portfolio | Mortgage Debt
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties disposed | property					4
Other Dispositions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Carrying value of mortgages assumed by buyer in real estate dispositions					$ 36,000,000
Other Dispositions | Mortgage Debt
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties disposed | property										1
Senior housing triple-net
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property															21
Proceeds from sale of buildings										$ 12,000,000			24,000,000	385,000,000	$ 428,000,000
Senior housing triple-net | Brookdale Triple Net Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings								510,000,000
Gain (loss) on sales of real estate, net								169,000,000
Senior housing triple-net | HRA Triple Net Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings							$ 132,000,000
Gain (loss) on sales of real estate, net							$ 33,000,000
Senior housing triple-net | Aegis NNN Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings									$ 358,000,000
Gain (loss) on sales of real estate, net									$ 228,000,000
SHOP
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings								230,000,000		$ 120,000,000		$ 12,000,000		$ 76,000,000	190,000,000
Payments to acquire finance receivables								150,000,000
Gain (loss) on sales of real estate, net								$ 59,000,000			$ 16,000,000
SHOP | Oakmont SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings					564,000,000
Gain (loss) on sales of real estate, net					80,000,000
SHOP | Discovery SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings					334,000,000
Gain (loss) on sales of real estate, net					9,000,000
Proceeds from sale of loans and preferred equity method investments					21,000,000
SHOP | Sonata SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings					64,000,000
Gain (loss) on sales of real estate, net					$ 3,000,000
SHOP | Atria SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property									12
Proceeds from sale of buildings									$ 312,000,000
Gain (loss) on sales of real estate, net									$ 39,000,000
SHOP | Other Dispositions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings													$ 169,000,000
Medical office
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings															136,000,000
MOB Land Parcels
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings															$ 3,000,000
SHOP
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property								16		8		4	15	13	23
SHOP | Sunrise Senior Housing Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property			2					32
Proceeds from sale of buildings								$ 664,000,000
Payments to acquire finance receivables								$ 410,000,000
Gain (loss) on sales of real estate, net													$ 7,000,000
SHOP | Oakmont SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property					12
SHOP | Discovery SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property					10
Gain (loss) on sales of real estate, net					$ 0
Number of preferred equity method investments sold | property					2
SHOP | Discovery SHOP Portfolio | Secured Loans
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of loans sold | property					2
SHOP | Sonata SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property					5
SHOP | SLC SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property				7
Repayments of secured debt				$ 70,000,000
SHOP | SLC SHOP Portfolio | Definitive Agreement Four
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of buildings				113,000,000
Gain (loss) on sales of real estate, net				$ 1,000,000
SHOP | Atria SHOP Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property									4
Payments to acquire finance receivables									$ 61,000,000
Senior housing triple-net
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property										4			7	18
Senior housing triple-net | Sunrise Senior Housing Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property			2
Proceeds from sale of buildings			$ 80,000,000
Gain (loss) on sales of real estate, net			$ 22,000,000
Senior housing triple-net | Brookdale Triple Net Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property								24
Senior housing triple-net | HRA Triple Net Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property							8
Senior housing triple-net | Aegis NNN Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property									10
Medical Office, Hospital
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property				1
Proceeds from sale of buildings				$ 226,000,000
Gain (loss) on sales of real estate, net				$ 172,000,000
Medical office
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property										3		4	7	7	11
Proceeds from sale of buildings										$ 36,000,000		$ 14,000,000	$ 57,000,000	$ 120,000,000
Medical office | San Diego
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property														3	3
MOB Land Parcels
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property												1		1	2
Proceeds from sale of buildings												$ 2,000,000		$ 2,000,000
Other Non-reportable Segments
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of assets to be sold | property												1		1	1
Proceeds from sale of buildings												$ 1,000,000		$ 1,000,000	$ 1,000,000