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Stock Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2013
Stock Based Compensation [Abstract] |  |
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, 2015, 2014, and 2013:
| 2015 | 2014 | 2013
Risk free rate - minimum | 0.10% | 0.02% | 0.07%
Risk free rate - maximum | 1.05% | 0.80% | 0.17%
Dividend yield | 3.932% | 4.755% | 4.269%
Volatility - minimum | 15.41% | 18.31% | 16.40%
Volatility - maximum | 16.04% | 20.48% | 20.92%
Service period | 3 years | 3 years | 4 years
Nonvested Restricted Stock Shares Activity [Table Text Block] | A summary of the status of the nonvested restricted shares as of December 31, 2015, and the changes for the year ended December 31, 2015, is presented below:
Nonvested Shares |  | Shares |  | Weighted Average Grant-Date Fair Value
Nonvested at January 1, 2015 |  | 145,049 |  |  | $ | 63.25 |
Issued |  | 93,356 |  |  | 72.54 |
Vested |  | (49,156 | ) |  | 58.08 |
Forfeited |  | (1,308 | ) |  | 65.88 |
Nonvested at December 31, 2015 |  | 187,941 |  |  | $ | 63.36 |
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 Black-Scholes valuation calculations for stock options granted during the year ended December 31, 2013:
|  | 2013
Term - minimum |  | 0.25 years
Term - maximum |  | 5.50 years
Risk free rate - minimum |  | 0.02%
Risk free rate - maximum |  | 1.55%
Dividend yield |  | 4.21%
Volatility - minimum |  | 15.60%
Volatility - maximum |  | 46.29%
Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block] | A summary of the status of the stock options as of December 31, 2015 and the changes for the year ended December 31, 2015 is presented below:
Stock Options |  | Options |  | Weighted Average Exercise Price
Outstanding at January 1, 2015 |  | 74,454 |  |  | $ | 82.33 |
Granted |  | — |  |  | — |
Exercised |  | (7,342 | ) |  | 57.23 |
Expired |  | (9,000 | ) |  | 76.87 |
Outstanding at December 31, 2015 |  | 58,112 |  |  | $ | 86.21 |

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- Definition Tabular disclosure of the changes in outstanding nonvested restricted stock shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the number and weighted-average exercise prices (or conversion ratios) for share options (or share units) that were outstanding at the beginning and end of the year, vested and expected to vest, exercisable or convertible at the end of the year, and the number of share options or share units that were granted, exercised or converted, forfeited, and expired during the year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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