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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 7 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
shares
Jun. 30, 2017
USD ($)
shares
Jun. 30, 2018
USD ($)
performance_metric
shares
Jun. 30, 2017
USD ($)
shares
Dec. 31, 2016
shares
Dec. 31, 2017
shares
Share-based compensation            
Number of units increased during the period (in shares) | shares           4
Number of units decreased during the period (in shares) | shares     8 57 3  
Recognized share-based compensation expense (benefit) | $ $ 0.5 $ 0.5 $ 0.6 $ 0.9    
Unrecognized compensation cost | $ $ 3.2   $ 3.2      
Nonvested shares outstanding (in shares) | shares 324 294 324 294 290 278
Weighted-average period over which unrecognized compensation cost is expected to be recognized (years)     2 years 4 months 24 days      
Accrued expenses and other liabilities            
Share-based compensation            
Accrued liability related to discretionary PRSUs | $ $ 0.1   $ 0.1      
Executive and Senior Officers            
Share-based compensation            
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            
Award vesting rights, percentage     25.00%      
Executive and Senior Officers | Anniversary 2            
Share-based compensation            
Award vesting rights, percentage     25.00%      
Executive and Senior Officers | Anniversary 3            
Share-based compensation            
Award vesting rights, percentage     25.00%      
Executive and Senior Officers | Anniversary 4            
Share-based compensation            
Award vesting rights, percentage     25.00%