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Share-Based Compensation and Benefit Plans (Black-Scholes Option Pricing Model) (Details) - Stock option [Member]
6 Months Ended
Jun. 30, 2018
Rate
Jun. 30, 2017
Rate
Share-Based Compensation and Benefit Plans    
Risk-free interest rate 2.59% 2.02%
Expected life 6 years 25 days 5 years 7 months 19 days
Expected volatility 23.80% 22.30%
Expected dividend yield 0.00% 0.00%