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Summary of Assumptions Used in Computing fair value of the PSUs (Parenthetical) (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Volatility [1]   27.00%
Dividend yield   8.10%
Expected term   2 years 9 months 18 days
Performance Share Units    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Volatility [2] 28.00%  
Dividend yield 8.20%  
Expected term 2 years 10 months 24 days  
Minimum | Performance Share Units | Peer Group and NAREIT    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Volatility 19.00% 19.00%
Maximum | Performance Share Units | Peer Group and NAREIT    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Volatility 41.00% 28.00%
[1] This represents the volatility assumption used for IRT. The volatility assumptions used for our peer group and the NAREIT Mortgage Index ranged from 19% to 28%.
[2] This represents the volatility assumption used for IRT. The volatility assumptions used for our peer group and the NAREIT Mortgage Index ranged from 19% to 41%.