Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions	12 Months Ended
Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value [Line Items]
Fair Value	$ 7,985.3	$ 7,481.8
Equity securities, at fair value	130.9	241.9
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.8	$ 3.6
Alternative Investment, Valuation Technique [Extensible Enumeration]	Discounted cash flow [Member]	Discounted cash flow [Member]
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Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Range	0.168	0.172
Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Range	0.168	0.172
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 1,185.2	$ 1,076.1
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 8.6	$ 9.3
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	6.10%	4.50%
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	6.70%	6.60%
Corporate [Member]
Fair Value [Line Items]
Fair Value	$ 3,849.7	$ 3,729.9
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.1	$ 0.1
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	$ 818.2	$ 836.4
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 7.0	$ 7.5
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 Rate For Lease Structure
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	3.00%	3.00%
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	3.00%	3.00%
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 Rate For Lease Structure
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Equity Securities [Member] \| Internal Price based on Financing Round [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 6.4	$ 6.4
Fair value measurement market multiples	0.27	0.27
Equity Securities [Member] \| Internal Price based on Financing Round [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	0.27	0.27
Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member] \| Net Tangible Asset [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.2	$ 1.1

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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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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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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