Amount of debt as of the reporting date.

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Amount of total gross properties and lease intangibles acquired at the acquisition date.

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The number of equity interests in unconsolidated commercial real estate properties.

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Represents the number of properties in the portfolio investment.

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The number of retail properties.

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The number of square feet properties.

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Represents the number of units in the portfolio investment.

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The number of units in a real estate property acquired during the period.

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Area of a real estate property.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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The net gain (loss) resulting from sales and other disposals of real estate owned for investment purposes.

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Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Cash received for the sale of real estate that is not part of an investing activity during the current period.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Properties - Narrative (Details) ft² in Millions, $ in Millions	1 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2017 Item	Sep. 30, 2017 ft² property	Sep. 30, 2021 USD ($) Item property	Sep. 30, 2020 USD ($)	Dec. 31, 2018 Item	Dec. 31, 2016 ft² Item	Dec. 31, 2015 Item
Real Estate Properties [Line Items]
Proceeds from sale of operating properties				$ 24.1
Operating properties
Real Estate Properties [Line Items]
Proceeds from sale of operating properties			$ 30.9
Gain on sale of property			9.7	7.4
Operating properties \| Commercial and Residential Lending Segment \| Montgomery Alabama
Real Estate Properties [Line Items]
Proceeds from sale of operating properties			30.6
Gain on sale of property			$ 17.7
Woodstar Portfolio
Real Estate Properties [Line Items]
Number of properties in portfolio investment \| Item			32
Number of units acquired \| Item			8,948
Number of acquired properties closed \| Item						14	18
Total gross properties and lease intangibles			$ 640.9
Debt			$ 573.4
Woodstar II Portfolio
Real Estate Properties [Line Items]
Number of properties in portfolio investment \| Item			27
Number of acquired properties closed \| Item	8				19
Total gross properties and lease intangibles			$ 611.8
Debt			$ 512.9
Number of units in portfolio investment \| Item			6,109
Medical Office Portfolio
Real Estate Properties [Line Items]
Number of acquired properties closed \| Item						34
Total gross properties and lease intangibles			$ 762.1
Debt			593.9
Area of property \| ft²						1.9
Master Lease Portfolio
Real Estate Properties [Line Items]
Total gross properties and lease intangibles			343.8
Debt			193.0
Number of retail properties acquired \| property		16
Number of square feet of properties \| ft²		1.9
Term of master lease agreements		24 years 7 months 6 days
Master Lease Portfolio \| Minimum \| Retail Properties \| Geographic Concentration Risk \| Utah, Florida, Texas and Minnesota
Real Estate Properties [Line Items]
Concentration risk (as a percent)		50.00%
REIS Equity Portfolio
Real Estate Properties [Line Items]
Total gross properties and lease intangibles			250.3
Debt			$ 176.7
Number of retail properties acquired \| property			14
Number of equity interests in unconsolidated commercial real estate properties \| property			1
REIS Equity Portfolio \| Operating properties
Real Estate Properties [Line Items]
Proceeds from sale of operating properties			$ 30.9	24.1
Gain on sale of property			$ 9.7	$ 7.4