XML 58 R47.htm IDEA: XBRL DOCUMENT v3.10.0.1
Income Taxes - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Sep. 30, 2018
Sep. 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 $ 1,000,000 $ 4,500,000        
Income tax expense 2,842,000   $ 2,673,000 7,205,000 $ 8,400,000  
Liabilities for uncertain tax positions $ 0 $ 0   $ 0   $ 0