Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 10,595
Held to Maturity, Due after one year through five years, Amortized Cost	47,873
Held to Maturity, Due after five years through ten years, Amortized Cost	519,679
Held to Maturity, Due after ten years, Amortized Cost	3,008,279
Held to Maturity, Amortized Cost	3,586,426	$ 3,619,738
Held to Maturity, Due in one year or less, Estimated Fair Value	10,542
Held to Maturity, Due after one year through five years, Estimated Fair Value	42,792
Held to Maturity, Due after five years through ten years, Estimated Fair Value	466,196
Held to Maturity, Due after ten years, Estimated Fair Value	2,897,407
Held to Maturity, Estimated Fair Value	3,416,937	3,532,044
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,350,261
Available for Sale, Due after one year through five years, Amortized Cost	2,871,230
Available for Sale, Due after five years through ten years, Amortized Cost	1,041,790
Available for Sale, Due after ten years, Amortized Cost	11,519,539
Debt Securities, Available-for-sale, Amortized Cost	16,825,791	17,965,338
Available for Sale, Due in one year or less, Estimated Fair Value	1,329,223
Available for Sale, Due after one year through five years, Estimated Fair Value	2,673,907
Available for Sale, Due after five years through ten years, Estimated Fair Value	964,649
Available for Sale, Due after ten years, Estimated Fair Value	10,186,888
Securities available for sale, at estimated fair value	15,197,638	16,578,371
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,173,271
Available for Sale, Due after one year through five years, Amortized Cost	2,621,114
Available for Sale, Due after five years through ten years, Amortized Cost	197,956
Available for Sale, Due after ten years, Amortized Cost	192,597
Debt Securities, Available-for-sale, Amortized Cost	4,184,938	5,212,894
Available for Sale, Due in one year or less, Estimated Fair Value	1,152,647
Available for Sale, Due after one year through five years, Estimated Fair Value	2,428,473
Available for Sale, Due after five years through ten years, Estimated Fair Value	163,742
Available for Sale, Due after ten years, Estimated Fair Value	137,469
Securities available for sale, at estimated fair value	3,882,331	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	43,600
Held to Maturity, Due after five years through ten years, Amortized Cost	467,780
Held to Maturity, Due after ten years, Amortized Cost	714,399
Held to Maturity, Amortized Cost	1,225,779	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	38,538
Held to Maturity, Due after five years through ten years, Estimated Fair Value	416,882
Held to Maturity, Due after ten years, Estimated Fair Value	695,506
Held to Maturity, Estimated Fair Value	1,150,926	1,196,332
Available-for-sale Securities, Due in one year or less, Amortized Cost	702
Available for Sale, Due after one year through five years, Amortized Cost	1,118
Available for Sale, Due after five years through ten years, Amortized Cost	13,084
Available for Sale, Due after ten years, Amortized Cost	7,552,891
Debt Securities, Available-for-sale, Amortized Cost	7,567,795	7,463,954
Available for Sale, Due in one year or less, Estimated Fair Value	696
Available for Sale, Due after one year through five years, Estimated Fair Value	1,121
Available for Sale, Due after five years through ten years, Estimated Fair Value	13,116
Available for Sale, Due after ten years, Estimated Fair Value	6,522,494
Securities available for sale, at estimated fair value	6,537,427	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	9,095
Held to Maturity, Due after one year through five years, Amortized Cost	4,273
Held to Maturity, Due after five years through ten years, Amortized Cost	51,899
Held to Maturity, Due after ten years, Amortized Cost	2,293,880
Held to Maturity, Amortized Cost	2,359,147	2,367,807
Held to Maturity, Due in one year or less, Estimated Fair Value	9,068
Held to Maturity, Due after one year through five years, Estimated Fair Value	4,254
Held to Maturity, Due after five years through ten years, Estimated Fair Value	49,314
Held to Maturity, Due after ten years, Estimated Fair Value	2,201,901
Held to Maturity, Estimated Fair Value	2,264,537	2,334,257
Available-for-sale Securities, Due in one year or less, Amortized Cost	176,288
Available for Sale, Due after one year through five years, Amortized Cost	248,998
Available for Sale, Due after five years through ten years, Amortized Cost	830,750
Available for Sale, Due after ten years, Amortized Cost	3,774,051
Debt Securities, Available-for-sale, Amortized Cost	5,030,087	5,245,721
Available for Sale, Due in one year or less, Estimated Fair Value	175,880
Available for Sale, Due after one year through five years, Estimated Fair Value	244,313
Available for Sale, Due after five years through ten years, Estimated Fair Value	787,791
Available for Sale, Due after ten years, Estimated Fair Value	3,526,925
Securities available for sale, at estimated fair value	4,734,909	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	1,474
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	1,474	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	42,971	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	$ 42,971	$ 42,769