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Benefit Plans - Shares Granted to Certain Employees and Trustees (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2017
Dec. 31, 2016
Index Performance Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares issued for services (in shares) 232,993 276,063    
Value per share granted (usd per share) $ 6.59 $ 5.05    
Peer Performance Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares issued for services (in shares) 232,987 276,058    
Value per share granted (usd per share) $ 5.97 $ 4.67    
Non-vested Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares issued for services (in shares) 243,270 277,460    
Value per share granted (usd per share) $ 7.77 $ 5.97    
Shares issued for services, value $ 2,581 $ 2,270    
Number of shares vested (in shares) 613,504 151,447    
Number of shares granted (in shares) 709,250 829,581    
Vesting period (in years) 3 years      
Performance Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Measurement period (in years) 3 years      
Number of shares vested (in shares) 122,779 713,044    
Number of shares granted (in shares)     452,737 808,929