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Stock Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Schedule of Share-based Payment Award, Restricted Stock, Valuation Assumptions [Table Text Block]

The following is a summary of the key assumptions used in the valuation calculations for market based awards granted during the years ended December 31, 2017, 2016 and 2015:



	2017	2016	2015
Risk free rate	0.65% - 1.57%	0.49% - 1.27%	0.10% - 1.05%
Dividend yield	3.573%	3.634%	3.932%
Volatility	20.43% - 21.85%	18.41% - 19.45%	15.41% - 16.04%
Requisite service period	3 years	3 years	3 years

Nonvested Restricted Stock Shares Activity [Table Text Block]

A summary of the status of the nonvested restricted shares as of December 31, 2017, and the changes for the year ended December 31, 2017, is presented below:



Nonvested Shares	Shares		Weighted Average Grant-Date Fair Value
Nonvested at January 1, 2017	225,624		$	71.61
Issued	106,113		87.09
Vested	(147,687	)	70.90
Forfeited	(3,358	)	84.97
Nonvested at December 31, 2017	180,692		$	81.13

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

The following is a summary of the key assumptions used in the Monte Carlo valuation calculations for stock options granted during the year ended December 31, 2016:



		2016
Risk free rate		0.64% - 2.63%
Dividend yield		3.81%
Volatility		21.02% - 21.57%
Expected term		1.11 - 2.11 years

Share-based Compensation, Stock Options, Activity [Table Text Block]

A summary of the status of the stock options as of December 31, 2017 and the changes for the year ended December 31, 2017 is presented below:



Stock Options	Options		Weighted Average Exercise Price
Outstanding at January 1, 2017	147,282		$	76.16
Granted	—		—
Exercised	(21,006	)	64.92
Expired	(17,838	)	109.05
Outstanding at December 31, 2017	108,438		$	72.93