Fair Value Measurements - Summary of Fair Value of Financial Instruments (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Financial assets:
Preferred equity investment - face value	$ 2,327,000	$ 4,531,000
Mortgage loans receivable
Financial assets:
Mortgage loans receivable - face value	23,701,000	12,375,000
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	295,532,000	295,153,000
Carrying Amount | Level 3
Financial assets:
Preferred equity investments	3,079,000	5,746,000
Carrying Amount | Level 3 | Mortgage loans receivable
Financial assets:
Mortgage loans receivable	23,646,000	12,299,000
Fair Value | Level 2 | Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable	311,625,000	289,500,000
Fair Value | Level 3
Financial assets:
Preferred equity investments	3,415,000	6,246,000
Fair Value | Level 3 | Mortgage loans receivable
Financial assets:
Mortgage loans receivable	$ 23,701,000	$ 12,375,000