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Employee Benefits Stock Based Incentive Plans (Details) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
2009 Stock Option and Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 4,000,000    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 2,414,842    
2010 Inducement Stock Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 1,245,000    
Share-based Compensation Arrangement by Share-based Payment Award, Terms of Award The terms of the Inducement Plan are substantially similar to the terms of the 2009 Plan.    
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period 24,569 37,177 22,936
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 727,491    
Employee Stock Purchase Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Terms of Award Under the ESPP, eligible employees may purchase common stock of the Company at 85 percent of the lower of the closing price of the Company’s common stock on the first or last day of a six month purchase period on The NASDAQ® Global Select Market. Employees pay for their stock purchases through payroll deductions at a rate equal to any whole percentage from 1 percent to 15 percent of after-tax earnings.    
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period 140,284    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 637,442    
Stock Options [Member] | 2009 Stock Option and Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Terms of Award Generally, options expire ten years from the date granted and vest over a three-year graded vesting period for officers and employees and a one-year or less period for non-employee directors.    
Restricted Stock [Member] | 2009 Stock Option and Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Terms of Award Stock grants generally vest over a three-year cliff vesting period