Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Narrative) (Detail) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 9,762.6	$ 8,466.7
Estimated fair value of debt securities, Unrealized loss position	9,421.6	$ 3,883.2
Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	102.7
Estimated fair value of debt securities, Unrealized loss position	53.7
Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	30.1
Estimated fair value of debt securities, Unrealized loss position	25.3
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	173.0
Estimated fair value of debt securities, Unrealized loss position	119.4
Non-Investment Grade \| Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	97.0
Estimated fair value of debt securities, Unrealized loss position	51.9
Non-Investment Grade \| High Yield Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	71.1
Estimated fair value of debt securities, Unrealized loss position	63.5
Non-Investment Grade \| Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	4.6
Estimated fair value of debt securities, Unrealized loss position	$ 3.7

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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+ Details

- 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.

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