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Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($)
$ in Thousands
Sep. 30, 2024
Dec. 31, 2023
Schedule of Investments [Line Items]    
Held to Maturity, Due in one year or less, Amortized Cost $ 9,108  
Held to Maturity, Due after one year through five years, Amortized Cost 408,753  
Held to Maturity, Due after five years through ten years, Amortized Cost 182,944  
Held to Maturity, Due after ten years, Amortized Cost 2,969,223  
Held to Maturity, Amortized Cost 3,570,028 $ 3,619,738
Held to Maturity, Due in one year or less, Estimated Fair Value 9,089  
Held to Maturity, Due after one year through five years, Estimated Fair Value 380,426  
Held to Maturity, Due after five years through ten years, Estimated Fair Value 170,483  
Held to Maturity, Due after ten years, Estimated Fair Value 2,938,273  
Held to Maturity, Estimated Fair Value 3,498,271 3,532,044
Available-for-sale Securities, Due in one year or less, Amortized Cost 1,288,771  
Available for Sale, Due after one year through five years, Amortized Cost 2,865,908  
Available for Sale, Due after five years through ten years, Amortized Cost 1,029,844  
Available for Sale, Due after ten years, Amortized Cost 11,222,244  
Debt Securities, Available-for-sale, Amortized Cost 16,449,841 17,965,338
Available for Sale, Due in one year or less, Estimated Fair Value 1,281,205  
Available for Sale, Due after one year through five years, Estimated Fair Value 2,744,175  
Available for Sale, Due after five years through ten years, Estimated Fair Value 979,783  
Available for Sale, Due after ten years, Estimated Fair Value 10,271,806  
Securities available for sale, at estimated fair value 15,320,043 16,578,371
US Treasury Securities [Member]    
Schedule of Investments [Line Items]    
Available-for-sale Securities, Due in one year or less, Amortized Cost 1,173,908  
Available for Sale, Due after one year through five years, Amortized Cost 2,623,994  
Available for Sale, Due after five years through ten years, Amortized Cost 198,031  
Available for Sale, Due after ten years, Amortized Cost 192,690  
Debt Securities, Available-for-sale, Amortized Cost 4,188,623 5,212,894
Available for Sale, Due in one year or less, Estimated Fair Value 1,166,473  
Available for Sale, Due after one year through five years, Estimated Fair Value 2,502,945  
Available for Sale, Due after five years through ten years, Estimated Fair Value 171,836  
Available for Sale, Due after ten years, Estimated Fair Value 146,492  
Securities available for sale, at estimated fair value 3,987,746 4,927,589
Residential Mortgage Backed Securities [Member]    
Schedule of Investments [Line Items]    
Held to Maturity, Due in one year or less, Amortized Cost 0  
Held to Maturity, Due after one year through five years, Amortized Cost 391,052  
Held to Maturity, Due after five years through ten years, Amortized Cost 131,149  
Held to Maturity, Due after ten years, Amortized Cost 689,597  
Held to Maturity, Amortized Cost 1,211,798 1,250,431
Held to Maturity, Due in one year or less, Estimated Fair Value 0  
Held to Maturity, Due after one year through five years, Estimated Fair Value 362,510  
Held to Maturity, Due after five years through ten years, Estimated Fair Value 120,099  
Held to Maturity, Due after ten years, Estimated Fair Value 696,937  
Held to Maturity, Estimated Fair Value 1,179,546 1,196,332
Available-for-sale Securities, Due in one year or less, Amortized Cost 232  
Available for Sale, Due after one year through five years, Amortized Cost 999  
Available for Sale, Due after five years through ten years, Amortized Cost 12,927  
Available for Sale, Due after ten years, Amortized Cost 7,404,639  
Debt Securities, Available-for-sale, Amortized Cost 7,418,797 7,463,954
Available for Sale, Due in one year or less, Estimated Fair Value 231  
Available for Sale, Due after one year through five years, Estimated Fair Value 1,015  
Available for Sale, Due after five years through ten years, Estimated Fair Value 13,011  
Available for Sale, Due after ten years, Estimated Fair Value 6,678,406  
Securities available for sale, at estimated fair value 6,692,663 6,596,682
US States and Political Subdivisions Debt Securities [Member]    
Schedule of Investments [Line Items]    
Held to Maturity, Due in one year or less, Amortized Cost 7,608  
Held to Maturity, Due after one year through five years, Amortized Cost 17,701  
Held to Maturity, Due after five years through ten years, Amortized Cost 51,795  
Held to Maturity, Due after ten years, Amortized Cost 2,279,626  
Held to Maturity, Amortized Cost 2,356,730 2,367,807
Held to Maturity, Due in one year or less, Estimated Fair Value 7,599  
Held to Maturity, Due after one year through five years, Estimated Fair Value 17,916  
Held to Maturity, Due after five years through ten years, Estimated Fair Value 50,384  
Held to Maturity, Due after ten years, Estimated Fair Value 2,241,336  
Held to Maturity, Estimated Fair Value 2,317,235 2,334,257
Available-for-sale Securities, Due in one year or less, Amortized Cost 114,631  
Available for Sale, Due after one year through five years, Amortized Cost 240,915  
Available for Sale, Due after five years through ten years, Amortized Cost 818,886  
Available for Sale, Due after ten years, Amortized Cost 3,624,915  
Debt Securities, Available-for-sale, Amortized Cost 4,799,347 5,245,721
Available for Sale, Due in one year or less, Estimated Fair Value 114,501  
Available for Sale, Due after one year through five years, Estimated Fair Value 240,215  
Available for Sale, Due after five years through ten years, Estimated Fair Value 794,936  
Available for Sale, Due after ten years, Estimated Fair Value 3,446,908  
Securities available for sale, at estimated fair value 4,596,560 5,011,331
Other Investments [Member]    
Schedule of Investments [Line Items]    
Held to Maturity, Due in one year or less, Amortized Cost 1,500  
Held to Maturity, Due after one year through five years, Amortized Cost 0  
Held to Maturity, Due after five years through ten years, Amortized Cost 0  
Held to Maturity, Due after ten years, Amortized Cost 0  
Held to Maturity, Amortized Cost 1,500 1,500
Held to Maturity, Due in one year or less, Estimated Fair Value 1,490  
Held to Maturity, Due after one year through five years, Estimated Fair Value 0  
Held to Maturity, Due after five years through ten years, Estimated Fair Value 0  
Held to Maturity, Due after ten years, Estimated Fair Value 0  
Held to Maturity, Estimated Fair Value 1,490 1,455
Available-for-sale Securities, Due in one year or less, Amortized Cost 0  
Available for Sale, Due after one year through five years, Amortized Cost 0  
Available for Sale, Due after five years through ten years, Amortized Cost 0  
Available for Sale, Due after ten years, Amortized Cost 0  
Debt Securities, Available-for-sale, Amortized Cost 43,074 42,769
Available for Sale, Due in one year or less, Estimated Fair Value 0  
Available for Sale, Due after one year through five years, Estimated Fair Value 0  
Available for Sale, Due after five years through ten years, Estimated Fair Value 0  
Available for Sale, Due after ten years, Estimated Fair Value 0  
Securities available for sale, at estimated fair value $ 43,074 $ 42,769