Fair Value Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 54.8	$ 28.6	$ 54.8	$ 43.4
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	73.6	25.0	73.7	25.0
Sales	(5.0)	0.0	(5.0)	0.0
Net Realized (Gain)/Loss on Sales	(4.5)	0.0	(4.5)	0.0
Change in Valuation	56.0	0.0	55.9	(14.8)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	174.9	53.6	174.9	53.6
Total equity securities \| Financials \| Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	35.0	13.1	35.0	27.1
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	60.2	25.0	60.2	25.0
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	33.7	0.0	33.7	(14.0)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	128.9	38.1	128.9	38.1
Total equity securities \| Industrials \| Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	16.6	15.2	16.7	16.0
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	5.0	0.0	5.0	0.0
Sales	(5.0)	0.0	(5.0)	0.0
Net Realized (Gain)/Loss on Sales	(4.5)	0.0	(4.5)	0.0
Change in Valuation	22.3	0.0	22.2	(0.8)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	34.4	15.2	34.4	15.2
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	3.2	0.3	3.1	0.3
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	8.4	0.0	8.5	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	$ 11.6	$ 0.3	$ 11.6	$ 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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- 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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- Definition

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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