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Stock-Based Compensation - Stock Option Plan (Details)
$ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Grant date fair value of stock options, vested $ 0.5
Intrinsic value of stock options 6.3
Stock-based compensation expense, unrecognized $ 0.6
Employee Stock Option [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock-based compensation expense, recognition period 1 year 4 months 13 days
Additional Paid-in Capital [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock-based compensation expense $ 1.3