Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Narrative) (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,054.4	$ 7,157.5
Estimated fair value of debt securities, Unrealized loss position	6,262.0	$ 5,879.6
Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	122.3
Estimated fair value of debt securities, Unrealized loss position	25.1
Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	39.4
Estimated fair value of debt securities, Unrealized loss position	32.7
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	187.1
Estimated fair value of debt securities, Unrealized loss position	76.0
Non-Investment Grade | Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	109.0
Estimated fair value of debt securities, Unrealized loss position	23.3
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	71.2
Estimated fair value of debt securities, Unrealized loss position	47.5
Non-Investment Grade | Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	5.2
Estimated fair value of debt securities, Unrealized loss position	$ 4.9