Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Narrative) (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,080.7	$ 7,157.5
Estimated fair value of debt securities, Unrealized loss position	6,314.9	$ 5,879.6
Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	120.6
Estimated fair value of debt securities, Unrealized loss position	20.7
Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	40.9
Estimated fair value of debt securities, Unrealized loss position	32.5
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	202.5
Estimated fair value of debt securities, Unrealized loss position	70.3
Non-Investment Grade | Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	110.8
Estimated fair value of debt securities, Unrealized loss position	19.6
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	84.5
Estimated fair value of debt securities, Unrealized loss position	46.1
Non-Investment Grade | Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	5.5
Estimated fair value of debt securities, Unrealized loss position	$ 4.3