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Debt Securities (Sales of Debt Securities) (Narrative) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract]        
Realized losses on sales of debt securities $ 5.2 $ 50.4 $ 18.8 $ 62.0
Realized gains on sales of debt securities 0.1 0.3 6.4 4.3
Proceeds from sales of debt securities $ 178.7 $ 605.4 $ 1,081.7 $ 1,062.8