Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Net Unrealized Gains on Investments, Net of Income Taxes:
No Credit Losses Recognized in Consolidated Statements of Income	$ 275.3	$ 439.4
Credit Losses Recognized in Consolidated Statements of Income	2.3	0.0
Net Unrecognized Postretirement Benefit Costs, Net of Income Taxes	(100.1)	(100.6)
Loss on Cash Flow Hedges, Net of Income Taxes	(2.6)	(2.7)
Accumulated Other Comprehensive Income	$ 174.9	$ 336.1

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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Amount, after tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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