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Income Taxes - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Income taxes $ (195,000) $ 368,000 $ 341,000
Percentage of ordinary income 100.00% 61.60% 83.80%
Capital gain distributions percentage 0.00% 34.40% 16.20%
Non-taxable distributions percentage 0.00% 4.00% 0.00%
Percentage of taxable income distributed among stockholders 90.00%