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, 2026 USD ($)	Dec. 31, 2025 USD ($)
Fair Value [Line Items]
Fair Value	$ 9,976.2	$ 9,536.5
Equity securities, at fair value	188.2	184.8
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.4	$ 3.4
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	0.186	0.186
Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Range	0.186	0.186
Municipals [Member]
Fair Value [Line Items]
Fair Value	$ 893.2	$ 847.7
Municipals [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 7.1	$ 7.4
Municipals [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	6.80%	6.80%
Municipals [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%
Corporates [Member]
Fair Value [Line Items]
Fair Value	$ 4,348.1	$ 4,253.5
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	854.9	589.9
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 4.8	$ 4.9
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	5.00%
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%
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		5.00%
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member] \| Measurement Input, Discount For Lease Structure [Member]
Fair Value [Line Items]
Significant Unobservable Inputs		0.30%
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	5.00%	5.00%
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 based on Financing Round [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 7.6	$ 7.3
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
Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member] \| Net Tangible Asset [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.4	$ 1.5

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