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Other Employee Benefit Plans (Tables) | 12 Months Ended
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Dec. 31, 2017
Other Employee Benefit Plans [Abstract] |
Schedule of shares purchased in the open market for employees | Shares purchased in the open market for employees for the years ended December 31, 2015, 2016, and 2017 were as follows:
|  | Shares |  | Average price |
|  | purchased |  | per share |
2015 |  | 5,136 |  | $ | 49.10 |
2016 |  | 4,988 |  | $ | 46.46 |
2017 |  | 4,718 |  | $ | 77.05 |

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- Definition Tabular disclosure of employee stock purchase plan activity. + 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 (g) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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