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Stock Based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2019
Share-based Payment Arrangement [Abstract] |
Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block] | During the six months ended June 30, 2019, the Company made the following awards of restricted stock:
Date |  | Shares Granted |  | Plan |  | Grant Date Fair Value Per Share |  | Vesting Period
2/21/2019 |  | 43,250 |  |  | 2005 Employee Stock Plan |  | $ | 83.87 |  |  | Ratably over 5 years from grant date
3/15/2019 |  | 600 |  |  | 2005 Employee Stock Plan |  | $ | 79.55 |  |  | Ratably over 5 years from grant date
4/1/2019 |  | 1,090 |  |  | 2005 Employee Stock Plan |  | $ | 82.62 |  |  | Ratably over 3 years from grant date
5/21/2019 |  | 6,500 |  |  | 2018 Non-Employee Director Stock Plan |  | $ | 77.08 |  |  | Shares vested immediately
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The Company did not grant any awards of options to purchase shares of common stock during the six months ended June 30, 2019.

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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=116856206&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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