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Retirement Plans and Employee Stock Purchase Plans (Tables) | 12 Months Ended
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Dec. 31, 2017
Retirement Plans and Employee Stock Purchase Plans |
Schedule of purchases made under the 2009 Employee Stock Purchase Plans |
|  | Year ended December 31, |
|  | 2017 |  | 2016 |  | 2015 |
Number of shares purchased |  |  | 88,362 |  |  | 151,531 |  |  | 254,098 |
Average price of shares purchased |  | $ | 33.96 |  | $ | 20.87 |  | $ | 13.50 |

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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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