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Debt Securities (Sales of Debt Securities) (Narrative) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]      
Realized losses on sales of debt securities $ 88.1 $ 141.4 $ 8.7
Realized gains on sales of debt securities 7.2 4.9 29.0
Proceeds from sales of debt securities $ 1,676.9 $ 1,753.3 $ 1,070.7