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Employee Stock Purchase Plan, Stock Options and Warrants - Fair Value Calculation Assumptions (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate, minimum 2.63% 1.77% 1.15%
Risk-free interest rate, maximum 2.77% 1.83% 1.40%
Expected option life 5 years 5 years 5 years
Expected dividend yield 0.00% 0.00% 0.00%
Expected price volatility, minimum 34.06% 33.81% 34.28%
Expected price volatility, maximum 34.32% 34.07% 37.06%