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Equity Incentive Plan - Additional Information (Details)
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Mar. 01, 2018
$ / shares
shares
Mar. 01, 2017
shares
Jan. 28, 2016
shares
Jun. 01, 2015
shares
May 17, 2013
shares
Jan. 31, 2018
shares
Jan. 31, 2017
shares
Sep. 30, 2018
USD ($)
shares
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
shares
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Common shares available for issuance (in shares)               1,405,529   1,405,529    
Independent Trustees                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Common share issued as compensation for services performed (in shares)           21,670 23,980          
Equity Incentive Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares authorized (in shares)         3,000,000              
Vesting period for share awards under equity         3 years              
Restricted Stock                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Unrecognized compensation costs | $               $ 43,100   $ 43,100   $ 100,000
Weighted-average period for recognition of unrecognized compensation costs                   1 year 3 months 19 days    
Compensation expense, recognized | $               8,400 $ 200,000 $ 100,000 $ 400,000  
Number of shares, Granted (in shares)                   0   5,000
Long Term Incentive Plan Units                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Weighted-average period for recognition of unrecognized compensation costs                   1 year 11 months 13 days    
Compensation expense, recognized | $               900,000 $ 700,000 $ 2,700,000 $ 1,800,000  
Exchange ratio                   1    
Number of shares, Granted (in shares)                   244,917   223,922
Total unrecognized compensation cost related to LTIP Units | $               $ 5,900,000   $ 5,900,000   $ 4,400,000
Long Term Incentive Plan Units | Awarded June 1, 2015                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Weighted-average period for recognition of unrecognized compensation costs                   2 years    
Number of shares, Granted (in shares)       183,300                
Long Term Incentive Plan Units, Time-Based                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares, Granted (in shares) 97,968 89,574 72,966                  
Long Term Incentive Plan Units, Time-Based | Awards March 1, 2018                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares, Granted (in shares) 97,968                      
Long Term Incentive Plan Units, Performance-Based                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares, Granted (in shares) 146,949 134,348 39,285                  
Long Term Incentive Plan Units, Performance-Based | Awards March 1, 2018                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Vesting period for share awards under equity 3 years                      
Number of shares, Granted (in shares) 146,949                      
Grants in period, intrinsic value, amount per share (in dollars per share) | $ / shares $ 17.02                      
Long Term Incentive Plan Units, Performance-Based | Awards March 1, 2018 | Minimum                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Percentage of units paid out 50.00%                      
Long Term Incentive Plan Units, Performance-Based | Awards March 1, 2018 | Maximum                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Percentage of units paid out 150.00%