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Stock-Based Compensation - 2018 Employee Stock Purchase Plan (Details) - Employee stock purchase plan
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2018
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Common stock purchased (in shares) | shares   0
2018 ESPP    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Percentage of employee earnings allowed 15.00% 15.00%
Discount price percentage   85.00%
Offering period (up to)   27 months
Employee contributions $ 2.3 $ 2.3
Unrecognized stock-based compensation expense $ 9.9 $ 9.9
Remaining offering period 1 year 10 months 24 days