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	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Underlying stock price at valuation date (in usd per share)	$ 113.27	$ 91.17
Expected volatility	30.30%	27.00%
Risk-free interest rate	1.30%	0.70%