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, 2025 USD ($)	Dec. 31, 2024 USD ($)
Fair Value [Line Items]
Fair Value	$ 8,808.3	$ 8,542.2
Equity securities, at fair value	161.0	157.7
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.9	$ 3.9
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	0.166	0.166
Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Range	0.166	0.166
Municipals [Member]
Fair Value [Line Items]
Fair Value	$ 883.9	$ 893.8
Municipals [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	7.6	7.6
Corporates [Member]
Fair Value [Line Items]
Fair Value	3,938.8	3,805.9
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	404.6	522.2
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	5.1	5.9
Equity Securities [Member] \| Internal Price based on Financing Round [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 7.4	$ 7.4
Fair value measurement market multiples	27	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	27	27
Net Tangible Asset [Member] \| Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.4	$ 1.4
Measurement Input, Discount for Small Issue Size [Member] \| Municipals [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.10%	6.10%
Measurement Input, Discount for Small Issue Size [Member] \| Municipals [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] \| Municipals [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.70%	6.70%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	5.00%
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%
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		5.00%
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	5.00%	4.70%
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%
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%
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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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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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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