Summary of Significant Accounting Policies - Taxes (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Business Acquisition [Line Items]
Net income (loss)	$ 1,368,000	$ (9,941,000)	$ 1,437,000	$ 2,391,000
State and local income, excise and franchise taxes	200,000	300,000	400,000	500,000
Liabilities for uncertain tax positions	0		0		$ 0
Real Estate Investment Trust
Business Acquisition [Line Items]
Net income (loss)	$ (200,000)	$ (20,000)	$ (200,000)	$ (31,000)