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Stock-Based Compensation Plans - Stock compensation expense (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock compensation expense | $ 2,122 | $ 1,660 | $ 3,387 | $ 2,585
2007 Stock Incentive Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock compensation expense | 1,143 | 1,660 | 2,402 | 2,565
2018 Stock Incentive Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock compensation expense | 975 | 0 | 975 | 0
Employee Stock Purchase Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock compensation expense | $ 4 | $ 0 | $ 10 | $ 20

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