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Other Employee Benefit Plans (Tables) | 12 Months Ended
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Dec. 31, 2019
Retirement Benefits [Abstract] |
Schedule of Shares Purchased in the Open Market For Employees | ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | Shares |  | Average price
​ | ​ | purchased | ​ | per share
2017 |  | 4,718 | ​ | $ | 77.05
2018 |  | 4,647 | ​ | $ | 100.34
2019 |  | 4,918 | ​ | $ | 126.83

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- Definition Tabular disclosure of employee stock purchase plan activity. + 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 (g) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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