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Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Dividend reinvestment plan, shares | 28,607 | 38,741 | 50,013
Stock purchase and compensation plans, shares | 635,521 | 720,323 | 863,426
Dividends to stockholders, per share | $ 0.66 | $ 0.61 | $ 0.57
Acquisition of treasury stock, shares | 136,205 | 152,595 | 147,461
Preferred stock, par value per share | $ 0 | $ 0 | $ 0
Preferred stock, shares authorized | 5,000,000 | 5,000,000 | 5,000,000
Preferred stock, shares issued | 0 | 0 | 0
Series A Preferred Stock [Member] |  |  |
Preferred stock, par value per share | $ 0 | $ 0 | $ 0
Preferred stock, shares authorized | 300,000 | 300,000 | 300,000

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- Definition Aggregate dividends declared during the period for each share of common stock outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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