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Schedule I - Summary Of Investments - Other Than Investments In Related Parties (Detail) $ in Millions | Dec. 31, 2023 USD ($)
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Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | $ 8,717.2 |
Market value | 7,971.9 |
Amount at which shown in the balance sheet | 7,971.3 |
Short-term Investments |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 55.8 |
Market value | 55.6 |
Amount at which shown in the balance sheet | 55.8 |
U.S. Treasury bonds |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 203.3 |
Market value | 199.3 |
Amount at which shown in the balance sheet | 199.3 |
Municipal bonds |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 1,373.7 |
Market value | 1,245.8 |
Amount at which shown in the balance sheet | 1,245.8 |
Foreign government bonds |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 228.4 |
Market value | 219.3 |
Amount at which shown in the balance sheet | 219.3 |
Governmental agency bonds |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 207.7 |
Market value | 195.4 |
Amount at which shown in the balance sheet | 195.4 |
Governmental agency mortgage-backed securities |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 4,396.2 |
Market value | 3,875.7 |
Amount at which shown in the balance sheet | 3,875.7 |
U.S. corporate debt securities |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 1,007.0 |
Market value | 958.4 |
Amount at which shown in the balance sheet | 958.4 |
Foreign corporate debt securities |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 478.9 |
Market value | 463.6 |
Amount at which shown in the balance sheet | 463.6 |
Debt Securities |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 7,895.2 |
Market value | 7,157.5 |
Amount at which shown in the balance sheet | 7,157.5 |
Equity Securities, Investment Summary |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 743.8 | [1]
Market value | 735.6 | [1]
Amount at which shown in the balance sheet | 735.6 | [1]
Notes Receivable |  |
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items] |  |
Cost | 22.4 |
Market value | 23.2 |
Amount at which shown in the balance sheet | $ 22.4 |
[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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