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Retirement Plans and Employee Stock Purchase Plans (Tables) | 12 Months Ended
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Dec. 31, 2020
Retirement Plans and Employee Stock Purchase Plans |
Schedule of purchases made under the 2009 Employee Stock Purchase Plans | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year ended December 31, |
​ | ​ | 2020 | ​ | 2019 | ​ | 2018 |
Number of shares purchased |  | ​ | 65,512 |  | ​ | 65,148 |  | ​ | 60,950 | ​
Average price of shares purchased | ​ | $ | 42.45 | ​ | $ | 48.58 | ​ | $ | 49.85 | ​

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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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