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Equity - Valuation Assumptions (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Equity [Abstract]      
Risk-free interest rate 1.84% 1.51% 1.08%
Expected life in years 5 years 5 years 5 years
Expected volatility 21.00% 24.00% 24.00%
Expected dividend yield 1.40% 1.36% 1.25%
Estimated fair value of options granted per share (in dollars per share) $ 10.50 $ 9.88 $ 8.19