Summary of Significant Accounting Policies (Valuation of Option Grants under Stock Option Plans) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	29.10%	30.60%	32.40%
Expected volatility, maximum	29.90%	31.40%	36.10%
Weighted-average volatility	29.90%	31.40%	36.10%
Risk-free rate, minimum	1.50%	2.70%	2.10%
Risk-free rate, maximum	2.40%	3.10%	2.30%
Expected term (in years)	15 years
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividends	2.36%	2.22%	1.98%
Expected term (in years)	7 years 4 months 24 days	7 years 4 months 24 days	7 years 4 months 24 days
Pre-vesting departure rate	0.60%	0.60%	0.60%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividends	2.95%	2.66%	2.28%
Expected term (in years)	7 years 4 months 24 days	7 years 4 months 24 days	7 years 4 months 24 days
Pre-vesting departure rate	0.60%	0.60%	0.60%