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Employee Stock Purchase Plan (Details) - Employee Stock - USD ($)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Employee Stock Purchase Plan    
Number of shares allowed to be purchased by employees (in shares) 1,000,000  
Purchase price of the shares with respect to market value (as a percent) 95.00%  
Maximum percentage of base compensation 15.00%  
Maximum market value $ 12,000  
Number of shares issued under the plan (in shares) 419 460