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Share-Based Compensation - Summary of Assumptions (Details) - 2007 Long-Term Incentive Plan
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Director and Officers      
Expected dividend yield 26.00% 22.00% 18.00%
Expected volatility 30.81% 41.19% 44.14%
Risk-free interest rate 1.90% 2.00% 1.97%
Expected life (in years) 5 years 7 years 8 months 5 days 8 years
Employees      
Expected dividend yield 26.00% 25.00% 22.00%
Expected volatility 30.67% 34.50% 42.71%
Risk-free interest rate 1.89% 1.73% 1.41%
Expected life (in years) 5 years 5 years 8 months 9 days 8 years