Debt Securities (Sales of Debt Securities) (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Debt Securities, Available-for-Sale [Line Items]
Realized losses on sales of debt securities	$ (346.9)	$ (5.2)	$ (352.4)	$ (18.8)
Realized gains on sales of debt securities	20.3	0.1	21.4	6.4
Proceeds from sales of debt securities	4,600.0	$ 178.7	$ 4,938.1	$ 1,081.7
Investment Portfolio Rebalancing Project
Debt Securities, Available-for-Sale [Line Items]
Realized losses on sales of debt securities	(345.4)
Proceeds from sales of debt securities	$ 2,800.0