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Accounting Policies (Accounting Pronouncements) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Impact of new accounting principle on prior period operating income | $ 81.5 | $ 66.9
Unfavorable impact on pre-tax operating results | (65.5) | (23.1)
Accounting Standards Update 2017-07 [Member] |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Impact of new accounting principle on prior period operating income |  | $ 13.6
Unfavorable impact on pre-tax operating results | $ 5.4 |

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