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Income Taxes - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Income Taxes [Line Items]            
Minimum Distribution Percentage of Taxable Income to Qualify for Real Estate Investment Trust       90.00%    
TCJA enacted date       Dec. 22, 2017    
U.S. federal corporate tax rate       21.00%   35.00%
TCJA one time transition tax       $ 0    
Effect of change in income tax associated with the ACT   $ 4,500,000        
Income tax expense $ 2,428,000   $ 3,242,000 4,363,000 $ 5,727,000  
Liabilities for uncertain tax positions $ 0 $ 0   $ 0   $ 0