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Debt Securities (Sales of Debt Securities) (Narrative) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Debt Securities, Available-for-Sale [Line Items]        
Realized losses on sales of debt securities $ 3.7 $ 346.9 $ 13.0 $ 352.4
Realized gains on sales of debt securities 1.7 20.3 4.2 21.4
Proceeds from sales of debt securities $ 240.4 $ 4,600.0 $ 747.4 $ 4,938.1