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Equity - Valuation Assumptions (Details) - $ / shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Equity [Abstract]    
Risk-free interest rate 2.66% 1.84%
Expected life in years 6 years 5 years
Expected volatility 20.00% 21.00%
Expected dividend yield 1.43% 1.40%
Estimated weighted-average fair value of options granted per share (in dollars per share) $ 13.98 $ 10.49