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Stock-Based Compensation - 2018 Performance Stock Units (Details) - Executive officers - Performance-based options - $ / shares
3 Months Ended 6 Months Ended
Mar. 31, 2018
Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of stock units granted (shares) 167,000 167,000
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting percentage 0.00%  
Weighted average fair value per stock units granted (usd per share) $ 10.09 $ 10.09
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting percentage 200.00%