Investments - Net Investment Gains (Losses) By Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ 0.0	$ 0.0	$ (1.1)	$ 0.0
Intent-to-sell impairments	0.0	(0.5)	(2.1)	(4.2)
Total impairments on investments recognized in net income	0.0	(0.5)	(3.2)	(4.2)
Sales and other, net	1.6	0.4	(0.5)	4.9
Change in fair value - equity securities	4.3	6.6	1.5	(7.9)
Change in fair value and losses realized on settlements - derivatives	(1.0)	(3.3)	(1.9)	(8.1)
Net investment gains (losses)	$ 4.9	$ 3.2	(4.1)	$ (15.3)
Available-for-sale allowance for credit loss not previously recorded			$ 1.1

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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