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, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value [Line Items]
Fair Value	$ 7,481.8	$ 7,723.9
Equity securities, at fair value	241.9	661.3
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.6	$ 4.3
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]
Significant Unobservable Inputs		16.10%
Range	0.172
Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs		16.10%
Range	0.172
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 1,076.1	$ 1,200.8
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 9.3	$ 14.0
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	4.50%	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	6.60%	6.50%
Corporate [Member]
Fair Value [Line Items]
Fair Value	$ 3,729.9	$ 4,090.1
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	$ 836.4	$ 824.4
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 7.5	$ 11.3
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%	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	3.00%	2.70%
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	$ 8.8
Fair value measurement market multiples	0.270
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	0.270
Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member] \| Net Tangible Asset [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.1	$ 1.3

Debt securities available for sale measurement input.

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Indicates valuation technique for measuring alternative investment.

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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), classified as current.

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