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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%

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- Definition Weighted average market capitalization percentage used to calculate the periodic increase/(decrease) in valuation of Level 3 securities. This value represents the minimum change in the range of valuation changes for the period. + References No definition available. + Details
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- Definition Weighted average market capitalization percentage used to calculate the periodic increase/(decrease) in valuation of Level 3 securities. This value represents the weighted average change for the period. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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