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Shareholders' Equity (Shares Repurchased Under Share Repurchase Program) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Class of Stock [Line Items] |  |  |  |
Aggregate purchase price |  |  | $ 100.0 |
Stock Repurchase Program Board Authorized Repurchases [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Shares repurchased (in shares) | 1.6 | 0.9 | 2.1 | 0.9
Aggregate purchase price | $ 73.4 | $ 34.9 | $ 100.0 | $ 34.9
Average purchase price per share (in dollars per share) | $ 47.30 | $ 39.92 | $ 47.58 | $ 39.92
Remaining amount available for share repurchase | $ 263.7 |  | $ 263.7 |

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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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