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Stock-Based Compensation - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Share-based Payment Arrangement [Abstract]        
Stock-based compensation expense not yet recognized $ 8,269   $ 8,269  
Stock-based compensation expense recognized $ 1,320 $ 1,216 $ 2,165 $ 2,084