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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, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period (in years) 5 years  
Index Performance Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares issued for services (in shares) 404,466 321,018
Value per share granted (usd per share) $ 4.53 $ 6.86
Peer Performance Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares issued for services (in shares) 404,463 321,011
Value per share granted (usd per share) $ 4.58 $ 6.66
Non-vested Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares issued for services (in shares) 225,090 170,650
Shares issued for services, value $ 1,724 $ 1,916
Value per share granted (usd per share) $ 5.23 $ 7.70
Vesting period (in years) 3 years  
Performance Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Measurement period (in years) 3 years  
Non-Management Award Grant [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares issued for services (in shares) 17,500 20,400
Shares issued for services, value $ 131 $ 209