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Share-Based Compensation - Assumptions Used to Determine Fair Value of Options Granted (Details) - Stock Options
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Assumptions used to determine fair value of options granted      
Dividend yield 0.50% 0.00% 0.00%
Expected volatility [1] 28.00% 30.70% 26.70%
Risk-free interest rate [2] 2.10% 1.60% 1.50%
Expected life of options (in years) [3] 5 years 8 months 12 days 5 years 8 months 12 days 5 years
Forfeiture rate 0.00% 0.00% 0.00%
[1] Expected volatility is based on historical and implied volatility.
[2] Risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant.
[3] Expected life of options (in years) is based on the Company’s historical and expected exercise behavior.