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Stock Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] | During the nine months ended September 30, 2018, the Company made the following awards of restricted stock:
Date |  | Shares Granted |  | Plan |  | Grant Date Fair Value Per Share |  | Vesting Period
2/15/2018 |  | 39,950 |  |  | 2005 Employee Stock Plan |  | $ | 71.75 |  |  | Ratably over 5 years from grant date
2/27/2018 |  | 1,150 |  |  | 2005 Employee Stock Plan |  | $ | 72.60 |  |  | Ratably over 5 years from grant date
5/15/2018 |  | 530 |  |  | 2005 Employee Stock Plan |  | $ | 74.00 |  |  | Ratably over 5 years from grant date
5/22/2018 |  | 6,000 |  |  | 2018 Non-Employee Director Stock Plan |  | $ | 76.58 |  |  | Shares vested immediately
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The Company has made the following awards of nonqualified options to purchase shares of common stock during the nine months ended September 30, 2018:
| Nine Months Ended September 30, 2018
Date of grant | 4/3/2018 |
Plan | 2010 Non-Employee Director Stock Plan |
Options granted | 5,000 |
Vesting period (1) | 21 months |
Expiration date | 4/3/2028 |
Expected volatility | 21.15 | %
Expected life (years) | 5.5 |
Expected dividend yield | 1.94 | %
Risk free interest rate | 2.62 | %
Fair value per option | $ | 13.46 |
(1) Vesting period began on the grant date.

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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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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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