Loans Receivable - Narrative (Details)		1 Months Ended								3 Months Ended					6 Months Ended	9 Months Ended		12 Months Ended
	Jan. 31, 2020 USD ($) property	Sep. 30, 2021 USD ($) property loan	Jul. 31, 2021 USD ($)	Jun. 30, 2021 USD ($) property	May 31, 2021 USD ($) property	Apr. 30, 2021 USD ($) property	Jan. 31, 2021 USD ($) property	Dec. 31, 2020 USD ($) property	Nov. 30, 2020 USD ($) property	Sep. 30, 2021 USD ($) property	Jun. 30, 2021 USD ($) property	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($) property	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) property	Sep. 30, 2021 USD ($) property	Sep. 30, 2020 USD ($) property	Dec. 31, 2020 USD ($) property	Dec. 31, 2019 USD ($)	Oct. 31, 2019 property
Loans Receivable:
Net cash provided by financing activities																$ (1,687,889,000)	$ 341,788,000
Gain (loss) on sales of real estate, net										$ 14,635,000			$ 2,283,000			$ 189,873,000	85,636,000
Number of loans held for sale | property		1								1						1
Financing receivable, after allowance for credit loss		$ 411,062,000						$ 195,375,000		$ 411,062,000						$ 411,062,000		$ 195,375,000
Reserves for loans receivable		$ 2,727,000						10,280,000		2,727,000						$ 2,727,000		$ 10,280,000	$ 0
Loss on sale of financing receivable														$ (42,000,000)
Number of loans sold		1														5		3
Financing receivable, held-for-sale		$ 51,000,000								51,000,000						$ 51,000,000
Property count | property	15
Proceeds from sale of receivables		2,000,000
Credit loss reserve on unfunded loan commitments		300,000						1,000,000		300,000						300,000		$ 1,000,000
Credit loss expenses										2,000,000			$ 3,000,000			(3,000,000)	$ (10,000,000)
Sunrise Senior Housing Portfolio | SHOP
Loans Receivable:
Net cash provided by financing activities							$ 410,000,000
Capital expenditure funding, amount committed				$ 56,000,000			$ 92,000,000
Capital expenditure funding, cost of capital, percent committed							65.00%
Capital expenditure funding, amount funded																400,000
Secured Loans
Loans Receivable:
Reserves for loans receivable		2,713,000						3,152,000		2,713,000						2,713,000		3,152,000	$ 0
Secured mortgage loans
Loans Receivable:
Financing receivable, gross		395,303,000						$ 161,530,000		$ 395,303,000						$ 395,303,000		$ 161,530,000
SHOP
Loans Receivable:
Number of assets to be sold | property							16			8			4			15	13	23
SHOP | Sunrise Senior Housing Portfolio
Loans Receivable:
Number of assets to be sold | property				2			32
Proceeds from sale of buildings							$ 664,000,000
Proceeds from the collection of loans receivable				$ 246,000,000
Gain (loss) on sales of real estate, net																$ 7,000,000
SHOP | Atria SHOP Portfolio
Loans Receivable:
Number of assets to be sold | property								4
SHOP | Discovery SHOP Portfolio
Loans Receivable:
Number of assets to be sold | property						10
Gain (loss) on sales of real estate, net						$ 0
SHOP | Atria SHOP Portfolio
Loans Receivable:
Number of assets to be sold | property								4
Proceeds from sale of buildings								$ 94,000,000
Net cash provided by financing activities								$ 61,000,000
SHOP | Secured Loans | Discovery SHOP Portfolio
Loans Receivable:
Number of loans held for sale | property						2
Loss on sale of financing receivable						$ 0
Number of loans sold | property						2
Other non-reportable
Loans Receivable:
Number of assets to be sold | property													1				1	1
Proceeds from sale of buildings													$ 1,000,000				$ 1,000,000	$ 1,000,000
Secured Loans
Loans Receivable:
Number of loans held for sale | property									1
Financing receivable, gross									$ 10,000,000
Loss on sale of financing receivable									2,000,000
Proceeds from sale of receivables									$ 8,000,000
Secured mortgage loans
Loans Receivable:
Number of loans held for sale | property				2				1			2				2			1
Financing receivable, gross				$ 64,000,000				$ 115,000,000			$ 64,000,000				$ 64,000,000			$ 115,000,000
Decrease in financing receivable															11,000,000
Financing receivable, after allowance for credit loss				53,000,000							53,000,000				53,000,000
Reserves for loans receivable				$ 8,000,000							8,000,000				$ 8,000,000
Loss on sale of financing receivable								6,000,000			$ 3,000,000					3,000,000
Principal repayments received		15,000,000	$ 8,000,000		$ 10,000,000
Number of loans receivable | property					1
Proceeds from sale of receivables								$ 109,000,000
Brookedale MTCA
Loans Receivable:
Gain (loss) on sales of real estate, net	$ 164,000,000
Lessor Asset Under Operating Lease | Other non-reportable
Loans Receivable:
Property count | property								1										1
Lessor Asset Under Operating Lease | Brookedale MTCA | Other non-reportable
Loans Receivable:
Property count | property	13
SHOP
Loans Receivable:
Proceeds from sale of buildings							230,000,000			$ 120,000,000			12,000,000				76,000,000	$ 190,000,000
Net cash provided by financing activities							150,000,000
Gain (loss) on sales of real estate, net							$ 59,000,000					$ 16,000,000
Loans receivable, market rate												5.25%
Non-cash interest income										1,000,000			$ 0			12,000,000	$ 0
Noncash interest income, acceleration recognition																7,000,000
SHOP | Minimum
Loans Receivable:
Loans receivable, stated interest rate												3.50%
Interest income, amortization period (in years)												1 year
SHOP | Maximum
Loans Receivable:
Loans receivable, stated interest rate												4.50%
Interest income, amortization period (in years)												3 years
SHOP | Atria SHOP Portfolio
Loans Receivable:
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 | Discovery SHOP Portfolio
Loans Receivable:
Proceeds from sale of buildings						$ 334,000,000
Gain (loss) on sales of real estate, net						$ 9,000,000
CCRC JV | Brookedale MTCA
Loans Receivable:
Property count | property	15
CCRC JV | Lessor Asset Under Operating Lease | Other non-reportable
Loans Receivable:
Property count | property																				15
CCRC JV | Lessor Asset Under Operating Lease | Brookedale MTCA | Other non-reportable
Loans Receivable:
Financing receivable, after allowance for credit loss	$ 30,000,000	$ 23,000,000						$ 23,000,000		$ 23,000,000						$ 23,000,000		$ 23,000,000
Property count | property	13