Annual report pursuant to Section 13 and 15(d)

Share-Based Compensation and Other Compensation Matters (Details 7)

v3.8.0.1
Share-Based Compensation and Other Compensation Matters (Details 7)
$ / shares in Units, $ in Thousands
12 Months Ended
Feb. 07, 2017
shares
Jan. 01, 2017
Percentile_Rank
$ / shares
Oct. 30, 2016
shares
Jul. 12, 2016
shares
May 30, 2016
shares
Mar. 05, 2015
shares
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2015
USD ($)
$ / shares
May 31, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Decrease in tax benefit from share-based compensation             $ 13 $ 331 $ 513  
Executive transition costs               6,500    
Stock Options [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Expiration period             10 years      
Performance share units [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Estimated pre-vesting forfeitures             0.00%      
Unrecognized compensation cost             $ 1,300      
Expected weighted average period during which unrecognized compensation cost will be recognized             2 years      
Performance period of the award             3 years      
The number of percentile ranks to fall between to earn interpolated PSUs between such percentile ranks, conditioned on the percentile rank exceeding 25% | Percentile_Rank   2                
Grant date fair value             $ 236      
Deferred Share Award [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Estimated pre-vesting forfeitures             0.00%      
Unrecognized compensation cost             $ 120      
Aggregate intrinsic value             $ 508 $ 322 $ 413  
Grant date fair value (in dollars per share) | $ / shares             $ 32.47 $ 26.89 $ 26.70  
Restricted shares [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Unrecognized compensation cost             $ 8,100      
Expected weighted average period during which unrecognized compensation cost will be recognized             3 years      
Aggregate intrinsic value             $ 5,300 $ 5,400 $ 4,900  
Grant date fair value (in dollars per share) | $ / shares             $ 33.84 $ 24.77 $ 28.69  
Restricted shares [Member] | Minimum [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Estimated pre-vesting forfeitures             0.00%      
Restricted shares [Member] | Maximum [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Estimated pre-vesting forfeitures             5.00%      
Options [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Aggregate intrinsic value of options exercised             $ 18   $ 300  
2017 Omnibus Equity and Incentive Plan [Member] | Maximum [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Number of common shares of beneficial interest authorized to be issued | shares                   3,400,000.0
2013 and 2014 PSU Grants [Member] | Performance share units [Member] | Former Chief Financial Officer [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares issued for PSU Awards Vested in Period (in shares) | shares           15,289        
2014 and 2015 Performance Share Unit Grants [Member] | Performance share units [Member] | Executive Vice President [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares issued for PSU Awards Vested in Period (in shares) | shares         10,326          
2014 and 2015 Performance Share Unit Grants [Member] | Performance share units [Member] | Chief Executive Officer [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares issued for PSU Awards Vested in Period (in shares) | shares       20,569            
2014, 2015 and 2016 Performance Share Unit Grants [Member] | Performance share units [Member] | General Counsel [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares issued for PSU Awards Vested in Period (in shares) | shares     2,248              
2017 PSU Grants [Member] | Performance share units [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Grant date fair value (in dollars per share) | $ / shares   $ 38.43                
2014 PSU Grants [Member] | Performance share units [Member] | Chief Executive Officer [Member]                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares issued for PSU Awards Vested in Period (in shares) | shares 9,763