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Stockholder's Equity - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Subsidiary, Sale of Stock [Line Items]        
Allocated share-based compensation expense $ 0.8 $ 1.4 $ 1.4 $ 2.0
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized $ 6.1 $ 1.5 $ 6.1 $ 1.5
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized, period for recognition     2 years 3 months 18 days  
Restricted stock        
Subsidiary, Sale of Stock [Line Items]        
Share-based compensation arrangement by share-based payment award, equity instruments other than options, grants in period (in shares) 44,424 102,571 371,875 166,410