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Assumptions Used in Valuation of Issued Stock Options (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]        
Risk-free interest rate 1.50% 2.00% 1.70% 2.00%
Expected life in years 7 years 6 years 3 months 18 days 6 years 9 months 18 days 6 years 3 months 18 days
Expected volatility 17.10% 17.20% 17.00% 17.20%
Dividend yield 1.40% 1.60% 1.50% 1.60%