Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Narrative) (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,454.5	$ 8,169.6
Estimated fair value of debt securities, Unrealized loss position	6,990.3	$ 7,645.8
Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	138.2
Estimated fair value of debt securities, Unrealized loss position	66.3
Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	45.1
Estimated fair value of debt securities, Unrealized loss position	39.9
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	175.9
Estimated fair value of debt securities, Unrealized loss position	107.3
Non-Investment Grade | Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	132.3
Estimated fair value of debt securities, Unrealized loss position	64.5
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	40.8
Estimated fair value of debt securities, Unrealized loss position	40.3
Non-Investment Grade | Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	2.5
Estimated fair value of debt securities, Unrealized loss position	$ 2.2