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 |