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Share-Based Compensation - Performance Restricted Stock Units - Narrative (Details) - PRSUs
shares in Thousands, $ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
shares
Jun. 30, 2018
USD ($)
shares
Jun. 30, 2019
USD ($)
performance_metric
shares
Jun. 30, 2018
USD ($)
shares
Dec. 31, 2018
shares
Dec. 31, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of units increased (decreased) during the period (in shares) | shares       (8) (3) 4
Current liability related to discretionary PSUs (less than) | $ $ 0.1   $ 0.1      
Recognized share-based compensation expense (benefit) | $ 0.4 $ 0.5 0.7 $ 0.6    
Unrecognized compensation cost | $ $ 3.7   $ 3.7      
Nonvested shares outstanding (in shares) | shares 387 324 387 324 277 278
Weighted-average period over which unrecognized compensation cost is expected to be recognized (years)     2 years 7 months 6 days      
Awards granted (in shares) | shares     190 123    
Executive and Senior Officers            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Performance period (years)     1 year      
Number of performance award metrics | performance_metric     3      
Requisite service period (years)     5 years      
Executive and Senior Officers | Anniversary 1            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting rights, percentage     25.00%      
Executive and Senior Officers | Anniversary 2            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting rights, percentage     25.00%      
Executive and Senior Officers | Anniversary 3            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting rights, percentage     25.00%      
Executive and Senior Officers | Anniversary 4            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting rights, percentage     25.00%