Federal Income Tax Reporting - General and Income Tax Expense (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Maximum ownership as a percentage of the voting power or value of the securities of each issuer other than REIT or "TRS"	10.00%
Maximum ownership of securities in all TRS (as a percent)	25.00%
Maximum ownership of securities in all TRS when considered together with other non-real estate assets (as a percent)	25.00%
Period of statute of limitations applicable to the entity's income tax returns	3 years
Net operating losses
NOLs expiration period	20 years
Gross amount of NOLs available to company	$ 13,041,000	$ 13,041,000	$ 13,041,000
Income Tax Expense
Revised Texas franchise tax	355,000	352,000	398,000
Other Taxes	45,000	66,000	35,000
Taxes on income	400,000	418,000	$ 433,000
Deferred income taxes	0
Real estate assets net tax basis more (less) than book basis	$ 208,434,000	$ 197,669,000