Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Narrative) (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,265.9	$ 7,157.5
Estimated fair value of debt securities, Unrealized loss position	6,175.0	$ 5,879.6
Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	139.7
Estimated fair value of debt securities, Unrealized loss position	17.3
Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	32.6
Estimated fair value of debt securities, Unrealized loss position	24.0
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	212.3
Estimated fair value of debt securities, Unrealized loss position	49.0
Non-Investment Grade | Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	128.4
Estimated fair value of debt securities, Unrealized loss position	17.0
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	75.4
Estimated fair value of debt securities, Unrealized loss position	27.5
Non-Investment Grade | Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	8.2
Estimated fair value of debt securities, Unrealized loss position	$ 4.2