Properties (Details) ft² in Millions	1 Months Ended		3 Months Ended		12 Months Ended			13 Months Ended	24 Months Ended
	Dec. 31, 2017 item	Sep. 30, 2017 ft² property	Mar. 31, 2021 USD ($) item property	Mar. 31, 2020 USD ($)	Dec. 31, 2018 item	Dec. 31, 2016 ft² item	Dec. 31, 2015 item	Dec. 31, 2018 item	Dec. 31, 2016 ft² item	Dec. 31, 2020 USD ($)
Summary of properties
Properties, cost			$ 2,565,472,000							$ 2,573,296,000
Less: accumulated depreciation			(320,724,000)							(302,143,000)
Properties, net			2,244,748,000							2,271,153,000
Proceeds from sale of operating properties				$ 0
Property Segment
Summary of properties
Land and land improvements			485,026,000							484,846,000
Buildings and building improvements			1,691,423,000							1,690,701,000
Furniture & fixtures			$ 60,926,000							59,632,000
Property Segment | Minimum
Summary of properties
Land and land improvements, useful life			0 years
Building and building improvements, useful life			5 years
Furniture & fixtures, useful life			3 years
Property Segment | Maximum
Summary of properties
Land and land improvements, useful life			15 years
Building and building improvements, useful life			45 years
Furniture & fixtures, useful life			7 years
Investing and Servicing Segment
Summary of properties
Land and land improvements			$ 50,617,000							50,585,000
Buildings and building improvements			179,813,000							179,014,000
Furniture & fixtures			$ 2,804,000							2,606,000
Investing and Servicing Segment | Minimum
Summary of properties
Land and land improvements, useful life			0 years
Building and building improvements, useful life			3 years
Furniture & fixtures, useful life			2 years
Investing and Servicing Segment | Maximum
Summary of properties
Land and land improvements, useful life			15 years
Building and building improvements, useful life			40 years
Furniture & fixtures, useful life			5 years
Commercial and Residential Lending Segment
Summary of properties
Land and land improvements			$ 9,691,000							11,416,000
Buildings			9,927,000							19,251,000
Construction in progress			$ 75,245,000							$ 75,245,000
Commercial and Residential Lending Segment | Minimum
Summary of properties
Land and land improvements, useful life			0 years
Building and building improvements, useful life			10 years
Commercial and Residential Lending Segment | Maximum
Summary of properties
Land and land improvements, useful life			7 years
Building and building improvements, useful life			20 years
Commercial and Residential Lending Segment | Operating properties
Summary of properties
Proceeds from sale of operating properties			$ 30,600,000
Gain on sale of property			$ 17,700,000
Woodstar Portfolio
Properties
Number of properties in portfolio investment | item			32						32
Total gross properties and lease intangibles			$ 636,700,000
Debt			$ 572,800,000
Number of units acquired | item			8,948
Number of acquired properties closed | item						14	18
Woodstar II Portfolio
Properties
Number of properties in portfolio investment | item			27
Total gross properties and lease intangibles			$ 610,600,000
Debt			$ 512,600,000
Number of units in portfolio investment | item			6,109
Number of acquired properties closed | item	8				19			27
Medical Office Portfolio
Properties
Area of property | ft²						1.9			1.9
Total gross properties and lease intangibles			$ 760,300,000
Debt			592,900,000
Number of acquired properties closed | item						34
Master Lease Portfolio
Properties
Total gross properties and lease intangibles			343,800,000
Debt			192,800,000
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
REIS Equity Portfolio
Properties
Total gross properties and lease intangibles			270,400,000
Debt			$ 192,600,000
Number of retail properties acquired | property			15
Number of equity interests in unconsolidated commercial real estate properties | property			1
Utah, Florida, Texas and Minnesota | Master Lease Portfolio | Geographic Concentration Risk | Minimum | Retail Properties [Member]
Properties
Concentration risk (as a percent)		50.00%