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Fair Value - Summary of Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2024
Jun. 30, 2024
Jun. 30, 2023
Dec. 31, 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Investments, fair value   $ 72,355.5 $ 59,265.6 $ 65,998.6
Fair Value, Inputs, Level 3        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Investments, fair value   101.3 112.0 110.1
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Investments, fair value   77.4 90.1 89.5
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Investments, fair value   23.9 21.9 20.6
Fair Value, Inputs, Level 3 | Corporate debt securities | Total Quantitative Level Three Securities        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Investments, fair value   $ 3.0   3.0
Fair value inputs, market cap price change percentage, range, minimum 0.30% (1.20%)    
Fair value inputs, market cap price change percentage, range, maximum 7.70% 1.20%    
Fair value inputs, market cap price change percentage, weighted average 2.60% 0.20%    
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Total Quantitative Level Three Securities        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Investments, fair value   $ 51.9 $ 71.8 64.0
Fair value inputs, market cap price change percentage, range, minimum 17.20% (7.60%) (6.10%)  
Fair value inputs, market cap price change percentage, range, maximum 39.70% (1.50%) 27.60%  
Fair value inputs, market cap price change percentage, weighted average 21.70% (2.90%) 4.40%  
Fair Value, Inputs, Level 3 | Common equities | Total Quantitative Level Three Securities        
Fair Value Measurement Inputs and Valuation Techniques [Line Items]        
Investments, fair value   $ 22.5 $ 18.3 $ 22.5
Fair value inputs, market cap price change percentage, range, minimum (45.80%) (26.50%) (22.00%)  
Fair value inputs, market cap price change percentage, range, maximum 95.60% 19.30% 125.40%  
Fair value inputs, market cap price change percentage, weighted average 39.70% (4.40%) 18.00%