Stock Options and Employee Stock Purchase Plan Stock Options and Employee Stock Purchase Plan (Market-Based Restricted Stock Awards, Valuation Assumptions) (Details) - Restricted (Performance) Stock - $ / shares	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Underlying stock price at valuation date (in usd per share)	$ 91.17	$ 84.58
Expected volatility	27.00%	28.30%
Risk-free interest rate	0.70%	2.53%