Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Narrative) (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,157.5	$ 8,169.6
Estimated fair value of debt securities, Unrealized loss position	5,879.6	$ 7,645.8
Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	130.5
Estimated fair value of debt securities, Unrealized loss position	24.6
Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	38.8
Estimated fair value of debt securities, Unrealized loss position	34.0
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	179.2
Estimated fair value of debt securities, Unrealized loss position	68.8
Non-Investment Grade | Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	120.3
Estimated fair value of debt securities, Unrealized loss position	24.2
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	53.6
Estimated fair value of debt securities, Unrealized loss position	41.1
Non-Investment Grade | Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	3.6
Estimated fair value of debt securities, Unrealized loss position	$ 3.2