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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, 2015
Dec. 31, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                              
Dividend yield                         0.40% 0.60% 0.60%
Expected volatility                         24.00% 33.00% 41.00%
Risk-free interest rate 0.11% 0.13% 0.09% 0.07% 0.15% 0.135% 0.12% 0.08% 0.12% 0.09% 0.06% 0.02%      
Expected life (years) 9 months 1 year 6 months 3 months 1 year 9 months 6 months 3 months 1 year 9 months 6 months 3 months