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Schedule I - Summary Of Investments - Other Than Investments In Related Parties (Detail) $ in Millions | Dec. 31, 2024 USD ($)
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Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | $ 8,564.5 |
Market value | 8,077.1 |
Amount at which shown in the balance sheet | 8,077.0 |
Short-term Investments |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 85.4 |
Market value | 85.3 |
Amount at which shown in the balance sheet | 85.4 |
U.S. Treasury bonds |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 180.7 |
Market value | 175.6 |
Amount at which shown in the balance sheet | 175.6 |
Municipal bonds |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 844.9 |
Market value | 776.5 |
Amount at which shown in the balance sheet | 776.5 |
Foreign government bonds |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 217.1 |
Market value | 211.6 |
Amount at which shown in the balance sheet | 211.6 |
Governmental agency bonds |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 203.8 |
Market value | 189.8 |
Amount at which shown in the balance sheet | 189.8 |
Governmental agency mortgage-backed securities |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 4,844.4 |
Market value | 4,502.3 |
Amount at which shown in the balance sheet | 4,502.3 |
U.S. corporate debt securities |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 948.4 |
Market value | 925.6 |
Amount at which shown in the balance sheet | 925.6 |
Foreign corporate debt securities |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 491.6 |
Market value | 484.5 |
Amount at which shown in the balance sheet | 484.5 |
Debt Securities |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 7,730.9 |
Market value | 7,265.9 |
Amount at which shown in the balance sheet | 7,265.9 |
Equity Securities, Investment Summary |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 713.8 | [1]
Market value | 691.3 | [1]
Amount at which shown in the balance sheet | 691.3 | [1]
Notes Receivable |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 34.4 |
Market value | 34.6 |
Amount at which shown in the balance sheet | $ 34.4 |
[1] | Included in equity securities are non-marketable equity securities and equity method investments, at carrying amount. Estimates of fair value for these investments could not be made without incurring excessive costs.

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