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Shareholders' Equity (Tables) | 12 Months Ended
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Dec. 31, 2017
Equity [Abstract] |
Summary of Activities in Common Stock | The following table details activity in our common stock:
| Years Ended December 31,
| 2017 |  | 2016 |  | 2015
| Shares |  | Amount |  | Shares |  | Amount |  | Shares |  | Amount
Stock option exercises | 1,113 |  |  | $ | 49,845 |  |  | 827 |  |  | $ | 23,527 |  |  | 1,118 |  |  | $ | 23,160 |
Purchases of common stock | (44 | ) |  | (6,613 | ) |  | (882 | ) |  | (112,699 | ) |  | (5 | ) |  | (645 | )
Employee stock plan purchases | 51 |  |  | 7,044 |  |  | 47 |  |  | 6,236 |  |  | 43 |  |  | 4,671 |
Shares issued for acquisitions | — |  |  | — |  |  | — |  |  | — |  |  | 2,149 |  |  | 364,333 |

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- Definition Tabular disclosure of changes in the separate accounts comprising stockholders' equity (in addition to retained earnings) and of the changes in the number of shares of equity securities during at least the most recent annual fiscal period and any subsequent interim period presented is required to make the financial statements sufficiently informative if both financial position and results of operations are presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 + Details
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