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Fair Value - Summary of Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, fair value $ 61,870.5 $ 52,330.4 $ 53,548.3
Fair Value, Inputs, Level 3      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, fair value 104.6 100.6 121.9
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, fair value 82.0 81.9 102.1
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, fair value 22.6 18.7 19.8
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 $ 63.7 $ 81.9 $ 83.8
Fair Value Inputs, Market Cap Price Change Percentage, Range, Minimum (10.80%) (20.70%) (0.60%)
Fair Value Inputs, Market Cap Price Change Percentage, Range, Maximum 4.50% (1.10%) 19.90%
Fair Value Inputs, Market Cap Price Change Percentage, Weighted Average 1.70% (6.60%) 10.50%
Fair Value, Inputs, Level 3 | Common equities | Total Quantitative Level Three Securities      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, fair value $ 18.3   $ 18.3
Fair Value Inputs, Market Cap Price Change Percentage, Range, Minimum (37.40%)   (42.50%)
Fair Value Inputs, Market Cap Price Change Percentage, Range, Maximum 130.20%   59.10%
Fair Value Inputs, Market Cap Price Change Percentage, Weighted Average (7.90%)   0.30%