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Net Income Per Share and Stockholders' Equity, Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Equity [Abstract] |  |
Repurchase of common stock (shares) | 58,155 | 415,346
Treasury Stock Acquired, Average Cost Per Share | $ 104.50 | $ 78.70
Treasury Stock, Value, Acquired, Cost Method | $ 6.1 | $ 32.7

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- Definition Total cost of shares repurchased divided by the total number of shares repurchased. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655 + Details
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