Fair Value Measurements - Summary of Fair Values of Financial Instruments (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Preferred equity investments - face value	$ 4,531,000	$ 12,031,000
Mortgage loan receivable - face value	12,399,000
Mortgage Loan Receivable
Financial assets:
Mortgage loan receivable - face value	12,517,000	0
Senior Unsecured Notes Payable
Financial liabilities:
Senior unsecured notes payable - face value	300,000,000	260,000,000
Carrying Amount | Level 2 | Senior Unsecured Notes Payable
Financial liabilities:
Senior unsecured notes payable	294,395,000	255,294,000
Carrying Amount | Level 3
Financial assets:
Preferred equity investments	5,550,000	13,872,000
Carrying Amount | Level 3 | Mortgage Loan Receivable
Financial assets:
Mortgage loan receivable	12,399,000	0
Fair Value | Level 2 | Senior Unsecured Notes Payable
Financial liabilities:
Senior unsecured notes payable	307,500,000	265,850,000
Fair Value | Level 3
Financial assets:
Preferred equity investments	5,423,000	14,289,000
Fair Value | Level 3 | Mortgage Loan Receivable
Financial assets:
Mortgage loan receivable	$ 12,517,000	$ 0