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Stock Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2017
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, 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 |

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- Definition Tabular disclosure of the changes in outstanding nonvested restricted stock shares. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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