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Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Jan. 01, 2020
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Investment in direct financing leases, net	$ 74,895,000		$ 74,895,000		$ 77,238,000
Impairments	756,000	$ 507,000	1,532,000	$ 1,538,000
Notes and mortgages receivable	$ 21,452,000		$ 21,452,000		11,280,000
Level 3 [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Assumed annual average rent increases for unobservable inputs			2.00%
Level 3 [Member] | Measurement Input Expected Term [Member] | Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Assumed holding periods for unobservable inputs			15 years
Level 3 [Member] | Measurement Input Cap Rate [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Real estate fair value, measurement input	8.00%		8.00%
Level 3 [Member] | Measurement Input Discount Rate [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Real estate fair value, measurement input	7.00%		7.00%
Properties [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments			$ 614,000	306,000
Estimated Sale Price Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments			43,000
Discounted Cash Flow Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments			0
Accumulation of Asset Retirement Cost Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments			1,489,000
Construction Loans [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Notes and mortgages receivable	$ 11,083,000		11,083,000
Notes and Mortgages Receivable [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments	0	0	0	0
Direct Financing Leases Financing Receivable [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments	0	$ 0	0	$ 0
Accounting Standards Update 2016-13 [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Investment in direct financing leases, net						$ 82,366,000
Estimated credit loss reserve related to direct financing leases						578,000
Additional allowance for credit losses					340,000
Credit loss reserve						309,000
Notes and mortgages receivable						$ 30,855,000
Proceeds from construction loans			11,083,000
Accounting Standards Update 2016-13 [Member] | Notes and Mortgages Receivable [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Additional allowance for credit losses	0		0		$ 28,000
Accounting Standards Update 2016-13 [Member] | Direct Financing Leases Financing Receivable [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Additional allowance for credit losses	$ 0		$ 0

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