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Share-Based Compensation (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Feb. 19, 2016
Jan. 15, 2016
Feb. 20, 2015
Jan. 16, 2015
Feb. 21, 2014
Jan. 17, 2014
Feb. 15, 2013
Feb. 17, 2012
Jan. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Compensation expense                   $ 1.0 $ 2.0 $ 1.5
Awards forfeited during period, value                   $ 0.2 $ 0.0 $ 0.0
Awards forfeited during period, units                   4,676 0 0
Outstanding awards vested during period                   $ 5.2    
Performance Units [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Cash payments distributed related to awards previously granted and now payable                 $ 1.3      
Performance Unit Plan [Member] | Performance Units [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period                   5 years    
Awards granted (in units)     18,948   21,991   22,884 25,040        
Performance Unit Plan [Member] | Performance Units [Member] | Subsequent Event [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Awards granted (in units) 19,055                      
Director Incentive Agreement [Member] | Performance Units [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period                   3 years    
Awards granted (in units)       3,794   4,683            
Director Incentive Agreement [Member] | Performance Units [Member] | Subsequent Event [Member]                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Awards granted (in units)   3,773