Fair Value Measurements - Summary of Fair Values of Financial Instruments (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Financial assets:
Preferred equity investments - face value	$ 4,531,000	$ 4,531,000
Mortgage Loan Receivable
Financial assets:
Mortgage loan receivable - face value	12,375,000	12,517,000
Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable - face value	300,000,000	300,000,000
Carrying Amount | Level 2 | Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable	295,153,000	294,395,000
Carrying Amount | Level 3
Financial assets:
Preferred equity investments	5,746,000	5,550,000
Carrying Amount | Level 3 | Mortgage Loan Receivable
Financial assets:
Mortgage loan receivable	12,299,000	12,399,000
Fair Value | Level 2 | Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable	289,500,000	307,500,000
Fair Value | Level 3
Financial assets:
Preferred equity investments	6,246,000	5,423,000
Fair Value | Level 3 | Mortgage Loan Receivable
Financial assets:
Mortgage loan receivable	$ 12,375,000	$ 12,517,000