Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Narrative) (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,624.7	$ 8,169.6
Estimated fair value of debt securities, Unrealized loss position	6,875.7	$ 7,645.8
Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	129.9
Estimated fair value of debt securities, Unrealized loss position	89.0
Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	45.5
Estimated fair value of debt securities, Unrealized loss position	38.9
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	172.6
Estimated fair value of debt securities, Unrealized loss position	133.2
Non-Investment Grade | Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	124.9
Estimated fair value of debt securities, Unrealized loss position	86.9
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	44.8
Estimated fair value of debt securities, Unrealized loss position	43.8
Non-Investment Grade | Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	2.6
Estimated fair value of debt securities, Unrealized loss position	$ 2.2