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Stockholders' Equity - ESPP (Details) - ESPP - $ / shares
6 Months Ended
Jun. 30, 2018
Jan. 01, 2018
Apr. 03, 2014
Share-based Compensation Arrangement by Share-based Payment Award      
Common stock reserved for future issuance (in shares)     880,000
Automatic annual increase in common stock reserved for issuance, percentage     1.00%
Maximum annual increase of shares reserved     1,000,000
Automatic annual increase in common stock reserved for issuance, shares   566,316  
Purchases made under the ESPP 136,321    
Weighted-average price per share $ 21.16