Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value [Line Items]
Fair Value	$ 7,370.3	$ 7,454.4
Equity securities, at fair value	641.0	598.5
Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.2	$ 1.1
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Fair Value	$ 4.0	$ 4.0
Alternative Investment, Valuation Technique [Extensible List]	Discounted cash flow [Member]	Discounted cash flow [Member]
Alternative Investment, Measurement Input [Extensible List]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Range	0.162	0.162
Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Range	0.162	0.162
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 1,073.1	$ 1,103.7
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 6.2	$ 7.0
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input Discount For Credit Stress [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.20%	0.20%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.70%	0.70%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.80%	6.80%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	5.00%	5.00%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount For Credit Stress [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.20%	0.20%
Corporate [Member]
Fair Value [Line Items]
Fair Value	$ 4,115.2	$ 4,111.3
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.4	$ 0.5
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.50%	2.50%
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.50%	2.50%
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	$ 756.5	$ 760.0
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 11.8	$ 12.4
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input Discount For Credit Stress [Member]
Fair Value [Line Items]
Significant Unobservable Inputs		0.20%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount For Lease Structure [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	1.90%	1.90%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3.10%	3.10%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.70%	2.70%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount For Credit Stress [Member]
Fair Value [Line Items]
Significant Unobservable Inputs		0.20%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount For Lease Structure [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Equity Securities [Member] \| Internal Price [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 4.4	$ 4.4
Fair value measurement market multiples	5.60	5.60
Equity Securities [Member] \| Internal Price [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	5.60	5.60
Equity Securities [Member] \| Market Comparables [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1.0	1.0
Equity Securities [Member] \| Market Comparables [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1.0	1.0

Debt securities available for sale measurement input.

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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Value of input used to measure alternative investment.

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Indicates measurement input for alternative investment.

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Indicates valuation technique for measuring alternative investment.

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Amount 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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Value of input used to measure investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Indicates measurement input for investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Indicates valuation technique for measuring investment in equity security at fair value with change in fair value recognized in net income (FV-NI).

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