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Net Income Per Share and Stockholders' Equity, Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017
Equity [Abstract] |  |
Number of common shares repurchased (shares) | 185,700 | 787,986
Weighted-average repurchase price (in usd per share) | $ 104.67 | $ 80.26
Total cost of repurchased common shares | $ 19.4 | $ 63.2

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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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