v3.26.1
Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2026
USD ($)
Dec. 31, 2025
USD ($)
Fair Value [Line Items]    
Fair Value $ 10,061.1 $ 9,536.5
Equity securities, at fair value 203.3 184.8
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]
Alternative Investment, Measurement Input [Extensible Enumeration] us-gaap:MeasurementInputDiscountRateMember us-gaap:MeasurementInputDiscountRateMember
Discounted cash flow [Member] | Level 3 [Member]    
Fair Value [Line Items]    
Range 0.186 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 $ 1,003.1 $ 847.7
Municipals [Member] | Discounted cash flow [Member] | Level 3 [Member]    
Fair Value [Line Items]    
Fair Value $ 6.7 $ 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 $ 4,515.2 $ 4,253.5
Commercial Mortgage-Backed [Member]    
Fair Value [Line Items]    
Fair Value 802.0 589.9
Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member]    
Fair Value [Line Items]    
Fair Value $ 4.7 $ 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