Investments, Net Realized Gains (Losses) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Change in unrealized gains (losses)
AFS, change in unrealized gains (losses) on debt securities reflected in AOCI	$ (69.0)	$ 63.1	$ 99.9
Total change in net unrealized gains (losses)	(34.1)	56.8	133.7
AFS, net realized gains (losses)
Total gross realized gains	25.3	51.9	23.0
Total gross realized losses	(6.1)	(25.9)	(5.7)
Net realized and unrealized gains on investments	54.6	19.0	51.1
Debt Securities, Available-for-sale, Allowance for Credit Loss, Period Increase (Decrease)	0.5	(0.7)	0.0
Proceeds from sale of fixed maturity securities	206.7	349.5	163.0
Debt Securities [Member]
Change in unrealized gains (losses)
AFS, change in unrealized gains (losses) on debt securities reflected in AOCI	(68.9)	63.0	99.9
AFS, net realized gains (losses)
Net realized and unrealized gains on investments	4.1	4.4	3.9
Debt Securities, Available-for-sale, Allowance for Credit Loss, Period Increase (Decrease)	0.5	(0.7)	0.0
Debt Securities, Available-for-sale, Realized Gain	4.7	9.2	5.2
Debt Securities, Available-for-sale, Realized Loss	1.1	4.1	1.3
Equity Securities [Member]
Change in unrealized gains (losses)
AFS, change in unrealized gains (losses) on debt securities reflected in AOCI	0.0	0.0	0.0
Equity securities, FV-NI, unrealized gain (loss)	30.0	(5.0)	33.1
Equity securities:
Equity securities, FV-NI, realized gain	20.6	42.6	17.8
Equity securities, FV-NI, realized loss	(5.0)	(21.8)	(4.4)
AFS, net realized gains (losses)
Net realized and unrealized gains on investments	45.6	15.8	46.5
Debt Securities, Available-for-sale, Allowance for Credit Loss, Period Increase (Decrease)	0.0	0.0	0.0
Other Investments [Member]
Change in unrealized gains (losses)
AFS, change in unrealized gains (losses) on debt securities reflected in AOCI	0.0	0.0	0.0
AFS, net realized gains (losses)
Net realized and unrealized gains on investments	4.9	(1.3)	0.7
Debt Securities, Available-for-sale, Allowance for Credit Loss, Period Increase (Decrease)	0.0	0.0	0.0
Unrealized Gain (Loss) on Investments	4.9	(1.3)	0.7
Loss on Sale of Investments	0.0	0.0	0.0
Gain on Sale of Investments	0.0	0.0	$ 0.0
Short-term Investments [Member]
Change in unrealized gains (losses)
AFS, change in unrealized gains (losses) on debt securities reflected in AOCI	(0.1)	0.1
AFS, net realized gains (losses)
Net realized and unrealized gains on investments	0.0	0.1
Debt Securities, Available-for-sale, Allowance for Credit Loss, Period Increase (Decrease)	0.0	0.0
Debt Securities, Available-for-sale, Realized Gain	0.0	0.1
Debt Securities, Available-for-sale, Realized Loss	$ 0.0	$ 0.0

The item represents the change in net unrealized gains (losses) for all fixed maturity and equity securities.

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Total gross realized gains for all fixed maturity and equity securities.

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Total gross realized losses for all fixed maturity and equity securities.

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Amount of increase (decrease) in allowance for credit loss of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain from sale of 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 realized loss from sale of 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 unrealized 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 realized and unrealized gain (loss) on investment.

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The difference between the carrying value and the sale price of an investment. A gain would be recognized when the sale price of the investment is greater than the carrying value of the investment. This element refers to the Gain included in earnings and not to the cash proceeds of the sale.

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The difference between the carrying value and the sale price of an investment. A loss would be recognized when the sale price of the investment is less than the carrying value of the investment. This element refers to the Loss included in earnings and not to the cash proceeds of the sale.

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Amount, before tax and after adjustment, of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and unrealized gain (loss) on investment in debt security measured at amortized cost (held-to-maturity) from transfer to available-for-sale.

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of unrealized gain (loss) on investment.

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