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Stockholders' Equity Stockholders' Equity (Four quarterly assumptions for fair value of the right (option) to purchase shares under ESPP) (Details) - Employee Stock Purchase Plan
3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                              
Dividend yield                         1.40% 1.40% 0.90%
Expected volatility                         36.00% 21.00% 19.00%
Risk-free interest rate 2.60% 2.56% 2.51% 2.42% 1.83% 1.72% 1.65% 1.44% 0.89% 0.77% 0.65% 0.53%      
Expected life (years) 1 year 9 months 6 months 3 months 1 year 9 months 6 months 3 months 1 year 9 months 6 months 3 months