Securities (Amortized Cost and Estimated Fair Value of Securities Excluding Trading Securities Presented by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 8,170
Held to Maturity, Due after one year through five years, Amortized Cost	13,508
Held to Maturity, Due after five years through ten years, Amortized Cost	565,101
Held to Maturity, Due after ten years, Amortized Cost	3,032,959
Held to Maturity, Amortized Cost	3,619,738	$ 2,639,241
Held to Maturity, Due in one year or less, Estimated Fair Value	8,175
Held to Maturity, Due after one year through five years, Estimated Fair Value	13,454
Held to Maturity, Due after five years through ten years, Estimated Fair Value	511,981
Held to Maturity, Due after ten years, Estimated Fair Value	2,998,434
Held to Maturity, Estimated Fair Value	3,532,044	2,467,865
Available for Sale, Due in one year or less, Amortized Cost	1,906,306
Available for Sale, Due after one year through five years, Amortized Cost	3,473,420
Available for Sale, Due after five years through ten years, Amortized Cost	1,092,453
Available for Sale, Due after ten years, Amortized Cost	11,450,390
Debt Securities, Available-for-sale, Amortized Cost	17,965,338	19,908,432
Available for Sale, Due in one year or less, Estimated Fair Value	1,886,056
Available for Sale, Due after one year through five years, Estimated Fair Value	3,286,388
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,040,857
Available for Sale, Due after ten years, Estimated Fair Value	10,322,301
Debt Securities, Available-for-sale	16,578,371	18,243,605
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Due in one year or less, Amortized Cost	1,534,307
Available for Sale, Due after one year through five years, Amortized Cost	3,288,365
Available for Sale, Due after five years through ten years, Amortized Cost	197,809
Available for Sale, Due after ten years, Amortized Cost	192,413
Debt Securities, Available-for-sale, Amortized Cost	5,212,894	5,450,546
Available for Sale, Due in one year or less, Estimated Fair Value	1,514,227
Available for Sale, Due after one year through five years, Estimated Fair Value	3,101,776
Available for Sale, Due after five years through ten years, Estimated Fair Value	167,219
Available for Sale, Due after ten years, Estimated Fair Value	144,367
Debt Securities, Available-for-sale	4,927,589	5,051,587
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [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	512,995
Held to Maturity, Due after ten years, Amortized Cost	737,436
Held to Maturity, Amortized Cost	1,250,431	526,122
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	461,253
Held to Maturity, Due after ten years, Estimated Fair Value	735,079
Held to Maturity, Estimated Fair Value	1,196,332	460,800
Available for Sale, Due in one year or less, Amortized Cost	1,740
Available for Sale, Due after one year through five years, Amortized Cost	1,827
Available for Sale, Due after five years through ten years, Amortized Cost	13,985
Available for Sale, Due after ten years, Amortized Cost	7,446,402
Debt Securities, Available-for-sale, Amortized Cost	7,463,954	7,316,824
Available for Sale, Due in one year or less, Estimated Fair Value	1,718
Available for Sale, Due after one year through five years, Estimated Fair Value	1,826
Available for Sale, Due after five years through ten years, Estimated Fair Value	13,963
Available for Sale, Due after ten years, Estimated Fair Value	6,579,175
Debt Securities, Available-for-sale	6,596,682	6,376,236
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	8,170
Held to Maturity, Due after one year through five years, Amortized Cost	12,008
Held to Maturity, Due after five years through ten years, Amortized Cost	52,106
Held to Maturity, Due after ten years, Amortized Cost	2,295,523
Held to Maturity, Amortized Cost	2,367,807	2,111,619
Held to Maturity, Due in one year or less, Estimated Fair Value	8,175
Held to Maturity, Due after one year through five years, Estimated Fair Value	11,999
Held to Maturity, Due after five years through ten years, Estimated Fair Value	50,728
Held to Maturity, Due after ten years, Estimated Fair Value	2,263,355
Held to Maturity, Estimated Fair Value	2,334,257	2,005,634
Available for Sale, Due in one year or less, Amortized Cost	370,259
Available for Sale, Due after one year through five years, Amortized Cost	183,228
Available for Sale, Due after five years through ten years, Amortized Cost	880,659
Available for Sale, Due after ten years, Amortized Cost	3,811,575
Debt Securities, Available-for-sale, Amortized Cost	5,245,721	7,098,635
Available for Sale, Due in one year or less, Estimated Fair Value	370,111
Available for Sale, Due after one year through five years, Estimated Fair Value	182,786
Available for Sale, Due after five years through ten years, Estimated Fair Value	859,675
Available for Sale, Due after ten years, Estimated Fair Value	3,598,759
Debt Securities, Available-for-sale	5,011,331	6,773,355
Other [Member]
Schedule of Available-for-Sale Securities [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	1,500
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	0
Held to Maturity, Due after one year through five years, Estimated Fair Value	1,455
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	1,455	1,431
Available for Sale, 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	42,769	42,427
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
Debt Securities, Available-for-sale	$ 42,769	$ 42,427