Investments - Net Investment (Losses) Gains By Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ (6.6)	$ 0.0	$ (7.7)	$ 0.0
Intent-to-sell impairments	0.0	(1.1)	(2.1)	(5.3)
Total impairments on investments recognized in net income	(6.6)	(1.1)	(9.8)	(5.3)
Sales and other, net	2.7	3.7	2.2	8.6
Change in fair value - equity securities	(1.1)	2.3	0.4	(5.6)
Change in fair value and losses realized on settlements - derivatives	(1.5)	(2.4)	(3.4)	(10.5)
Net investment (losses) gains	$ (6.5)	$ 2.5	(10.6)	$ (12.8)
Available-for-sale allowance for credit loss not previously recorded			$ 7.7

Other than Temporary Impairment Losses, Investments, Portion Recognized In Earnings, Change In Intent

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Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Amount of unrealized and realized gain (loss) on investment in debt and equity securities.

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Amount of expense for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), for which no credit loss was previously recorded.

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Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings.

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Amount of other-than-temporary impairment (OTTI) on investment in debt security, recognized in earnings.

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Amount of realized gain (loss) on investment.

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