Schedule IV - Mortgage Loans on Real Estate - Mortgage Loans on Real Estate (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Carrying amount		$ 14,448	$ 21,838	$ 23,197	$ 23,824
Mortgage Loans [Member]
Face amount		20,392
Carrying amount	[1]	13,993
Other Financing Loans [Member]
Face amount		21,167
Carrying amount	[1]	$ 14,448
Retail Site [Member] | Westport, CT [Member] | Borrower A [Member] | Mortgage Loans [Member]
Interest rate		6.50%
Face amount		$ 5,014
Carrying amount	[1]	$ 5,014
Retail Site [Member] | Las Vegas, NV [Member] | Borrower B [Member] | Mortgage Loans [Member]
Interest rate		12.00%
Face amount		$ 3,075
Carrying amount	[1]	$ 3,075
Retail Site [Member] | Miami, FL [Member] | Borrower C [Member] | Mortgage Loans [Member]
Interest rate		7.57%
Face amount		$ 4,201
Carrying amount	[1]	$ 1,646
Retail Site [Member] | Miami, FL [Member] | Borrower D [Member] | Mortgage Loans [Member]
Interest rate		7.57%
Face amount		$ 3,678
Carrying amount	[1]	$ 1,613
Nonretail [Member] | Mortgage Loans [Member] | Individually Less Than 3% [Member]
Interest rate	[2],[3]
Face amount	[2]	$ 2,474
Carrying amount	[1],[2]	$ 695
Nonretail [Member] | Other Financing Loans [Member] | Individually Less Than 3% [Member]
Interest rate	[4],[5]
Face amount	[4]	$ 775
Carrying amount	[1],[4]	$ 455
Nonretail [Member] | Oakbrook Terrace, IL [Member] | Borrower E [Member] | Mortgage Loans [Member]
Interest rate		6.00%
Face amount		$ 1,950
Carrying amount	[1]	$ 1,950

[1] The aggregate cost for Federal income tax purposes was approximately $14.4 million as of December 31, 2018.

[2] Comprised of four separate loans with original loan amounts ranging from $0.5 million to $1.4 million.

[3] Interest rates range from 6.88% to 7.41%.

[4] Comprised of two separate loans with original loan amounts ranging from $0.2 million to $0.6 million.

[5] Interest rates range from 2.28% to 6.85%.