Mortgage Notes Receivable - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage notes receivable, net		$ 20,294,000	$ 22,764,000
Reserve for loan loss	$ 15,300,000	0	0	$ 15,300,000
Principal repayments received from borrowers		$ 6,796,000	5,417,000	$ 6,921,000
Mortgage notes receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of mortgage notes receivable | loan		8
Weighted-average interest rate		6.20%
Weighted-average years to maturity		12 years 7 months 21 days
Number of mortgage notes receivable repaid at maturity | loan		1
Mortgage notes receivable, net		$ 20,294,000	1,500,000
Gain (Loss) on early repayment of loan receivable		$ 100,000
Number of mortgage notes with capitalized principal and interest | loan		1
Unsecured note reserve [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal repayments received from borrowers		$ 0	$ 0