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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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