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Share-Based Compensation (Details) - USD ($)
$ in Millions
12 Months Ended
Feb. 15, 2019
Jan. 18, 2019
Mar. 01, 2018
Feb. 16, 2018
Jan. 19, 2018
Mar. 01, 2017
Jan. 20, 2017
Feb. 19, 2016
Feb. 20, 2015
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Compensation expense                   $ 1.1 $ 1.0 $ 3.1
Cash payments distributed related to awards previously granted and now payable                   1.6    
Awards forfeited during period, value                   $ 0.0 $ 0.0 $ 0.0
Awards forfeited during period, units                   0 0 0
Outstanding awards vested during period                   $ 5.2    
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)       17,830   14,704   19,055 18,948      
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) 17,022                      
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)     1,106   4,704   4,608          
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)   5,175