Real Estate Transactions - Dispositions of Real Estate (Details) $ in Thousands		1 Months Ended	3 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	Jan. 31, 2020 USD ($) property	Oct. 31, 2020 USD ($) property	Sep. 30, 2020 USD ($) property	Jun. 30, 2020 USD ($) property	Mar. 31, 2020 USD ($) property	Sep. 30, 2019 USD ($) property	Jun. 30, 2021 USD ($) property	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($) property	Dec. 31, 2019 property
Real Estate [Line Items]
Gain (loss) on sales of real estate, net			$ 149		$ 165,000	$ (784)		$ 247,881	$ 18,708
Number of properties disposed | property					18
Brookedale MTCA
Real Estate [Line Items]
Gain (loss) on sales of real estate, net	$ 164,000
Number of properties disposed | property	18									18
SHOP and Senior Housing Triple-Net | Subsequent Event
Real Estate [Line Items]
Proceeds from sale of buildings		$ 88,000
SHOP
Real Estate [Line Items]
Proceeds from sale of buildings			12,000	$ 28,000	$ 36,000	7,000			89,000
Gain (loss) on sales of real estate, net				2,000
SHOP | Subsequent Event | Definitive Agreement Four
Real Estate [Line Items]
Proceeds from sale of buildings		$ 3,000					$ 115,000
Senior housing triple-net
Real Estate [Line Items]
Proceeds from sale of buildings					$ 385,000				26,000
Medical office
Real Estate [Line Items]
Proceeds from sale of buildings			14,000			$ 3,000			18,000
Medical office | San Diego
Real Estate [Line Items]
Proceeds from sale of buildings				106,000
Gain (loss) on sales of real estate, net				$ 81,000
Undeveloped MOB land parcel
Real Estate [Line Items]
Proceeds from sale of buildings			$ 2,000
Life science
Real Estate [Line Items]
Proceeds from sale of buildings									7,000
Undeveloped Life Science
Real Estate [Line Items]
Proceeds from sale of buildings									$ 35,000
SHOP
Real Estate [Line Items]
Number of assets to be sold | property			4	2	7	1			11
SHOP | Subsequent Event
Real Estate [Line Items]
Number of assets to be sold | property		7
SHOP | Subsequent Event | Definitive Agreement Four
Real Estate [Line Items]
Number of assets to be sold | property							7
Senior housing triple-net
Real Estate [Line Items]
Number of assets to be sold | property					18				2
Senior housing triple-net | Subsequent Event
Real Estate [Line Items]
Number of assets to be sold | property		3
Medical office
Real Estate [Line Items]
Number of assets to be sold | property			4			1			6
Medical office | San Diego
Real Estate [Line Items]
Number of assets to be sold | property				3
Undeveloped MOB land parcel
Real Estate [Line Items]
Number of assets to be sold | property			1
Other non-reportable
Real Estate [Line Items]
Number of assets to be sold | property			1
Proceeds from sale of buildings			$ 1,000
Life science
Real Estate [Line Items]
Number of assets to be sold | property									1
Undeveloped Life Science
Real Estate [Line Items]
Number of assets to be sold | property									1