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Accounting Policies (Accounting Pronouncements) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Mar. 31, 2016 | Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Unfavorable impact on pre-tax operating results |  | $ (23.1) | $ 132.3 |
Accounting Standards Update 2017-07 [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Projected impact of new accounting principle on prior period operating income |  |  |  | $ 70.6
Forecast | Accounting Standards Update 2017-07 [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Unfavorable impact on pre-tax operating results | $ 5.0 |  |  |

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