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Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2024 USD ($) | Dec. 31, 2023 USD ($)
Fair Value [Line Items] |  |
Fair Value | $ 8,620.0 | $ 7,985.3
Equity securities, at fair value | 150.1 | 130.9
Level 3 [Member] |  |
Fair Value [Line Items] |  |
Fair Value | $ 3.8 | $ 3.8
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 | 0.168 | 0.168
Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] |  |
Fair Value [Line Items] |  |
Range | 0.168 | 0.168
Municipals [Member] |  |
Fair Value [Line Items] |  |
Fair Value | $ 1,049.3 | $ 1,185.2
Municipals [Member] | Discounted cash flow [Member] | Level 3 [Member] |  |
Fair Value [Line Items] |  |
Fair Value | 8.0 | 8.6
Corporates [Member] |  |
Fair Value [Line Items] |  |
Fair Value | 3,973.9 | 3,849.7
Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member] |  |
Fair Value [Line Items] |  |
Fair Value |  | 0.1
Commercial Mortgage-Backed [Member] |  |
Fair Value [Line Items] |  |
Fair Value | 586.2 | 818.2
Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] |  |
Fair Value [Line Items] |  |
Fair Value | 6.2 | 7.0
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 | 27
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 | 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 | $ 1.2
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] | Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member] |  |
Fair Value [Line Items] |  |
Significant Unobservable Inputs |  | 2.50%
Measurement Input, Discount for Small Issue Size [Member] | Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] |  |
Fair Value [Line Items] |  |
Significant Unobservable Inputs |  | 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 | 5.00% | 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 | 4.70% | 3.00%
Measurement Input, Discount For Above-market coupon [Member] | Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member] |  |
Fair Value [Line Items] |  |
Significant Unobservable Inputs |  | 0.30%
Measurement Input, Discount For Above-market coupon [Member] | Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] |  |
Fair Value [Line Items] |  |
Significant Unobservable Inputs |  | 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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