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, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value [Line Items]
Fair Value	$ 7,678.2	$ 7,481.8
Equity securities, at fair value	234.8	241.9
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.6	$ 3.6
Alternative Investment, Valuation Technique [Extensible Enumeration]	Discounted cash flow [Member]	Discounted cash flow [Member]
Alternative Investment, Measurement Input [Extensible Enumeration]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Range	0.172	0.172
Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Range	0.172	0.172
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 1,125.8	$ 1,076.1
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	9.1	9.3
Corporate [Member]
Fair Value [Line Items]
Fair Value	3,779.9	3,729.9
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	0.1	0.1
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	842.5	836.4
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	7.5	7.5
Equity Securities [Member] \| Internal Model Pricing Approach Valuation Technique \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 6.4	$ 6.4
Fair value measurement market multiples	27.0	27.0
Equity Securities [Member] \| Internal Model Pricing Approach Valuation Technique \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	27.0	27.0
Net Tangible Asset [Member] \| Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.1	$ 1.1
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	4.50%	4.50%
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.80%	6.80%
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.60%	6.60%
Measurement Input, Discount for Small Issue Size [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.50%	2.50%
Measurement Input, Discount for Small Issue Size [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.50%	2.50%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3.00%	3.00%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3.10%	3.10%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3.00%	3.00%
Measurement Input, Discount For Above-market coupon [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Above-market coupon [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Above-market coupon [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Measurement Input, Discount For Above-market coupon [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Measurement Input, Discount For Lease Structure [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Lease Structure [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%

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