Schedule IV - Mortgage Loans on Real Estate (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate [Line Items]
Interest Rate		10.00%	9.90%
Prior Liens		$ 0
Face Amount of Mortgages		30,429
Carrying Amount of Mortgages	[1]	30,429
Principal Amount of Loans Subject to delinquent Principal or Interest		0
Mortgage Loans on Real Estate, Other Required Disclosures [Abstract]
Aggregate tax basis for Federal tax purposes		30,400	$ 29,900
Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at beginning of year		29,904	41,618	$ 50,988
Issuance of loans		525		368
Collection and satisfaction of loans			(11,714)	(10,692)
Amortization of discount				954
Balance at end of year		30,429	$ 29,904	$ 41,618
Mortgage On Retail Buildings in Philadelphia, PA, Loan One
Mortgage Loans on Real Estate [Line Items]
Prior Liens		0
Face Amount of Mortgages		21,179
Carrying Amount of Mortgages	[2]	21,179
Principal Amount of Loans Subject to delinquent Principal or Interest		0
Mortgage On Retail Buildings in Philadelphia, PA, Loan One | Maximum
Mortgage Loans on Real Estate [Line Items]
Face Amount of Mortgages		$ 25,000
Mortgage On Retail Buildings in Philadelphia, PA, Loan Two
Mortgage Loans on Real Estate [Line Items]
Interest Rate		10.00%
Prior Liens		$ 0
Face Amount of Mortgages		9,250
Carrying Amount of Mortgages		9,250
Principal Amount of Loans Subject to delinquent Principal or Interest		$ 0
Interest Rate One | Mortgage On Retail Buildings in Philadelphia, PA, Loan One
Mortgage Loans on Real Estate [Line Items]
Interest Rate		8.00%
Interest Rate Two | Mortgage On Retail Buildings in Philadelphia, PA, Loan One
Mortgage Loans on Real Estate [Line Items]
Interest Rate		10.00%

[1] For Federal tax purposes, the aggregate tax basis is approximately $30.4 million as of December 31, 2017.

[2] This mortgage is available for up to $25.0 million.