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Stockholders' Equity (Stock Purchase) (Details) - 2014 Employee Stock Purchase Plan - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Oct. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Purchase period of offering 6 months      
Rate of purchase price of stock on fair value (as a percent) 85.00%      
Number of shares reserved for future issuance 770,549      
Rate of increase in the number of shares available for grant every year on outstanding common stock (as a percent) 1.00%      
Number of shares available for future grants 495,551      
Purchases under the award 108,081 122,667    
Weighted Average purchase price $ 5.98 $ 6.14    
Stock-based compensation expense $ 0.4 $ 0.3 $ 0.4  
Maximum        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Discount rate on the value of shares through payroll deductions (as a percent) 15.00%      
Expiration period of each offering 27 months      
Number of shares reserved for future issuance       255,500
Number of shares available for future grants 3,000,000