Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 10,631
Held to Maturity, Due after one year through five years, Amortized Cost	4,281
Held to Maturity, Due after five years through ten years, Amortized Cost	563,809
Held to Maturity, Due after ten years, Amortized Cost	3,022,318
Held to Maturity, Amortized Cost	3,601,039	$ 3,619,738
Held to Maturity, Due in one year or less, Estimated Fair Value	10,558
Held to Maturity, Due after one year through five years, Estimated Fair Value	4,268
Held to Maturity, Due after five years through ten years, Estimated Fair Value	506,223
Held to Maturity, Due after ten years, Estimated Fair Value	2,930,267
Held to Maturity, Estimated Fair Value	3,451,316	3,532,044
Available-for-sale Securities, Due in one year or less, Amortized Cost	729,453
Available for Sale, Due after one year through five years, Amortized Cost	3,527,430
Available for Sale, Due after five years through ten years, Amortized Cost	1,059,756
Available for Sale, Due after ten years, Amortized Cost	11,408,116
Debt Securities, Available-for-sale, Amortized Cost	16,767,625	17,965,338
Available for Sale, Due in one year or less, Estimated Fair Value	715,897
Available for Sale, Due after one year through five years, Estimated Fair Value	3,315,269
Available for Sale, Due after five years through ten years, Estimated Fair Value	995,014
Available for Sale, Due after ten years, Estimated Fair Value	10,112,533
Securities available for sale, at estimated fair value	15,181,583	16,578,371
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	499,348
Available for Sale, Due after one year through five years, Amortized Cost	3,291,567
Available for Sale, Due after five years through ten years, Amortized Cost	197,882
Available for Sale, Due after ten years, Amortized Cost	192,504
Debt Securities, Available-for-sale, Amortized Cost	4,181,301	5,212,894
Available for Sale, Due in one year or less, Estimated Fair Value	486,430
Available for Sale, Due after one year through five years, Estimated Fair Value	3,082,345
Available for Sale, Due after five years through ten years, Estimated Fair Value	164,133
Available for Sale, Due after ten years, Estimated Fair Value	139,664
Securities available for sale, at estimated fair value	3,872,572	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	0
Held to Maturity, Due after five years through ten years, Amortized Cost	511,806
Held to Maturity, Due after ten years, Amortized Cost	727,614
Held to Maturity, Amortized Cost	1,239,420	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	0
Held to Maturity, Due after five years through ten years, Estimated Fair Value	455,963
Held to Maturity, Due after ten years, Estimated Fair Value	714,388
Held to Maturity, Estimated Fair Value	1,170,351	1,196,332
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,348
Available for Sale, Due after one year through five years, Amortized Cost	1,272
Available for Sale, Due after five years through ten years, Amortized Cost	13,807
Available for Sale, Due after ten years, Amortized Cost	7,455,768
Debt Securities, Available-for-sale, Amortized Cost	7,472,195	7,463,954
Available for Sale, Due in one year or less, Estimated Fair Value	1,333
Available for Sale, Due after one year through five years, Estimated Fair Value	1,275
Available for Sale, Due after five years through ten years, Estimated Fair Value	13,793
Available for Sale, Due after ten years, Estimated Fair Value	6,456,224
Securities available for sale, at estimated fair value	6,472,625	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	9,131
Held to Maturity, Due after one year through five years, Amortized Cost	4,281
Held to Maturity, Due after five years through ten years, Amortized Cost	52,003
Held to Maturity, Due after ten years, Amortized Cost	2,294,704
Held to Maturity, Amortized Cost	2,360,119	2,367,807
Held to Maturity, Due in one year or less, Estimated Fair Value	9,092
Held to Maturity, Due after one year through five years, Estimated Fair Value	4,268
Held to Maturity, Due after five years through ten years, Estimated Fair Value	50,260
Held to Maturity, Due after ten years, Estimated Fair Value	2,215,879
Held to Maturity, Estimated Fair Value	2,279,499	2,334,257
Available-for-sale Securities, Due in one year or less, Amortized Cost	228,757
Available for Sale, Due after one year through five years, Amortized Cost	234,591
Available for Sale, Due after five years through ten years, Amortized Cost	848,067
Available for Sale, Due after ten years, Amortized Cost	3,759,844
Debt Securities, Available-for-sale, Amortized Cost	5,071,259	5,245,721
Available for Sale, Due in one year or less, Estimated Fair Value	228,134
Available for Sale, Due after one year through five years, Estimated Fair Value	231,649
Available for Sale, Due after five years through ten years, Estimated Fair Value	817,088
Available for Sale, Due after ten years, Estimated Fair Value	3,516,645
Securities available for sale, at estimated fair value	4,793,516	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	1,466
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	1,466	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	42,870	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	$ 42,870	$ 42,769