Fair Value Measurements - Summary of Fair Value of Financial Instruments (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Financial assets:
Preferred equity investment - face value	$ 4,531,000	$ 4,531,000
Mortgage Loan Receivable
Financial assets:
Mortgage loans receivable - face value	12,483,000	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	294,584,000	294,395,000
Carrying Amount | Level 3
Financial assets:
Preferred equity investments	5,645,000	5,550,000
Carrying Amount | Level 3 | Mortgage Loan Receivable
Financial assets:
Mortgage loans receivable	12,494,000	12,399,000
Fair Value | Level 2 | Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable	302,250,000	307,500,000
Fair Value | Level 3
Financial assets:
Preferred equity investments	5,610,000	5,423,000
Fair Value | Level 3 | Mortgage Loan Receivable
Financial assets:
Mortgage loans receivable	$ 12,483,000	$ 12,517,000