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Quarterly Financial Data (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Jun. 30, 2017 | Jun. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Quarterly Financial Data [Line Items] |  |  |  |  |  |  |  |  |  |
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount |  |  |  |  | $ 161.1 |  |  | $ 0.0 | $ 161.1 | $ 0.0
Loss on early extinguishment of long-term debt |  |  | $ 33.0 |  |  | $ 111.5 | $ 12.5 | (45.5) | $ (111.5) | $ 0.0
Interest Rate Swap Settled |  |  |  |  |  |  |  |  |  |
Quarterly Financial Data [Line Items] |  |  |  |  |  |  |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | [1] | $ 25.0 |  | $ 21.2 |  |  |  | 46.2 |  |
Columbia Of Massachusetts |  |  |  |  |  |  |  |  |  |
Quarterly Financial Data [Line Items] |  |  |  |  |  |  |  |  |  |
Expenses Related to Third Party Claims and Other Expenses |  | $ 426.0 | $ 462.0 |  |  |  |  |  |  |
Federal tax on excess stock compensation [Domain] |  |  |  |  |  |  |  |  |  |
Quarterly Financial Data [Line Items] |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Income Taxes |  |  |  |  |  |  |  | $ 120.7 |  |
[1] | See Note 9, "Risk Management Activities" for additional information.

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