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Summary of Significant Accounting Policies (Valuation of Option Grants under Stock Option Plans) (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility, minimum 30.60% 32.40% 37.10%
Expected volatility, maximum 31.40% 36.10% 43.10%
Weighted-average volatility 31.40% 36.10% 41.00%
Risk-free rate, minimum 2.70% 2.10% 1.40%
Risk-free rate, maximum 3.10% 2.30% 2.10%
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected dividends 2.22% 1.98% 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%
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected dividends 2.66% 2.28% 2.94%
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%