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Share-Based Compensation (Details) - USD ($)
$ in Millions
12 Months Ended
Feb. 16, 2018
Jan. 19, 2018
Mar. 01, 2017
Jan. 20, 2017
Feb. 19, 2016
Jan. 15, 2016
Feb. 20, 2015
Feb. 21, 2014
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Compensation expense                 $ 1.0 $ 3.1 $ 1.0
Awards forfeited during period, value                 $ 0.0 $ 0.0 $ 0.2
Awards forfeited during period, units                 0 0 4,676
Outstanding awards vested during period                 $ 5.9    
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                 $ 1.7    
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)     14,704   19,055   18,948 21,991      
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) 17,830                    
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)       4,608   3,773          
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)   4,704