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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, 2025 USD ($) | Dec. 31, 2024 USD ($)
Fair Value [Line Items] |  |
Fair Value | $ 9,536.5 | $ 8,542.2
Equity securities, at fair value | 184.8 | 157.7
Level 3 [Member] |  |
Fair Value [Line Items] |  |
Fair Value | $ 3.4 | $ 3.9
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 | 0.166
Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] |  |
Fair Value [Line Items] |  |
Range | 0.186 | 0.166
Municipals [Member] |  |
Fair Value [Line Items] |  |
Fair Value | $ 847.7 | $ 893.8
Municipals [Member] | Discounted cash flow [Member] | Level 3 [Member] |  |
Fair Value [Line Items] |  |
Fair Value | $ 7.4 | $ 7.6
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% | 6.10%
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%
Municipals [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%
Corporates [Member] |  |
Fair Value [Line Items] |  |
Fair Value | $ 4,253.5 | $ 3,805.9
Commercial Mortgage-Backed [Member] |  |
Fair Value [Line Items] |  |
Fair Value | 589.9 | 522.2
Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] |  |
Fair Value [Line Items] |  |
Fair Value | $ 4.9 | $ 5.9
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 Above-market coupon [Member] |  |
Fair Value [Line Items] |  |
Significant Unobservable Inputs |  | 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%
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 |  | 5.00%
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% | 4.70%
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%
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 | $ 7.3 | $ 7.4
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 | $ 1.5 | $ 1.4

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