Fair Value Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 53.6	$ 32.4	$ 43.4	$ 30.4
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	2.4	0.0	27.4	2.0
Sales	0.0	0.0	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	0.0
Change in Valuation	0.0	0.0	(14.8)	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	56.0	32.4	56.0	32.4
Total equity securities \| Financials \| Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	38.1	27.1	27.1	25.1
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	25.0	2.0
Sales	0.0	0.0	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	0.0
Change in Valuation	0.0	0.0	(14.0)	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	38.1	27.1	38.1	27.1
Total equity securities \| Industrials \| Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	15.2	5.0	16.0	5.0
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	0.0
Change in Valuation	0.0	0.0	(0.8)	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	15.2	5.0	15.2	5.0
Total equity securities \| Other risk investments \| Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	0.3	0.3	0.3	0.3
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	2.4	0.0	2.4	0.0
Sales	0.0	0.0	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	0.0
Change in Valuation	0.0	0.0	0.0	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	$ 2.7	$ 0.3	$ 2.7	$ 0.3

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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- Definition

Amount of increase (decrease) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

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