Securities (Amortized Cost and Estimated Fair Value of Securities Excluding Trading Securities Presented by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 10,585
Held to Maturity, Due after one year through five years, Amortized Cost	525,201
Held to Maturity, Due after five years through ten years, Amortized Cost	90,168
Held to Maturity, Due after ten years, Amortized Cost	2,805,725
Held to Maturity, Amortized Cost	3,431,679	$ 3,534,085
Held to Maturity, Due in one year or less, Estimated Fair Value	10,597
Held to Maturity, Due after one year through five years, Estimated Fair Value	497,818
Held to Maturity, Due after five years through ten years, Estimated Fair Value	88,178
Held to Maturity, Due after ten years, Estimated Fair Value	2,698,838
Held to Maturity, Estimated Fair Value	3,295,431	3,360,546
Available for Sale, Due in one year or less, Amortized Cost	1,086,664
Available for Sale, Due after one year through five years, Amortized Cost	1,717,442
Available for Sale, Due after five years through ten years, Amortized Cost	862,545
Available for Sale, Due after ten years, Amortized Cost	13,304,644
Debt Securities, Available-for-sale, Amortized Cost	17,013,723	16,602,158
Available for Sale, Due in one year or less, Estimated Fair Value	1,080,998
Available for Sale, Due after one year through five years, Estimated Fair Value	1,649,609
Available for Sale, Due after five years through ten years, Estimated Fair Value	814,980
Available for Sale, Due after ten years, Estimated Fair Value	12,382,581
Debt Securities, Available-for-sale	15,970,596	15,043,625
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Due in one year or less, Amortized Cost	845,957
Available for Sale, Due after one year through five years, Amortized Cost	1,367,327
Available for Sale, Due after five years through ten years, Amortized Cost	198,405
Available for Sale, Due after ten years, Amortized Cost	193,163
Debt Securities, Available-for-sale, Amortized Cost	2,604,852	3,692,215
Available for Sale, Due in one year or less, Estimated Fair Value	840,252
Available for Sale, Due after one year through five years, Estimated Fair Value	1,299,741
Available for Sale, Due after five years through ten years, Estimated Fair Value	176,274
Available for Sale, Due after ten years, Estimated Fair Value	140,250
Debt Securities, Available-for-sale	2,456,517	3,442,320
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	495,067
Held to Maturity, Due after five years through ten years, Amortized Cost	11,221
Held to Maturity, Due after ten years, Amortized Cost	607,186
Held to Maturity, Amortized Cost	1,113,474	1,193,840
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	467,368
Held to Maturity, Due after five years through ten years, Estimated Fair Value	9,734
Held to Maturity, Due after ten years, Estimated Fair Value	613,104
Held to Maturity, Estimated Fair Value	1,090,206	1,122,764
Available for Sale, Due in one year or less, Amortized Cost	64
Available for Sale, Due after one year through five years, Amortized Cost	9,847
Available for Sale, Due after five years through ten years, Amortized Cost	2,424
Available for Sale, Due after ten years, Amortized Cost	8,805,804
Debt Securities, Available-for-sale, Amortized Cost	8,818,139	8,024,704
Available for Sale, Due in one year or less, Estimated Fair Value	63
Available for Sale, Due after one year through five years, Estimated Fair Value	9,838
Available for Sale, Due after five years through ten years, Estimated Fair Value	2,485
Available for Sale, Due after ten years, Estimated Fair Value	8,109,408
Debt Securities, Available-for-sale	8,121,794	6,997,902
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	10,585
Held to Maturity, Due after one year through five years, Amortized Cost	28,634
Held to Maturity, Due after five years through ten years, Amortized Cost	78,947
Held to Maturity, Due after ten years, Amortized Cost	2,198,539
Held to Maturity, Amortized Cost	2,316,705	2,338,745
Held to Maturity, Due in one year or less, Estimated Fair Value	10,597
Held to Maturity, Due after one year through five years, Estimated Fair Value	28,951
Held to Maturity, Due after five years through ten years, Estimated Fair Value	78,444
Held to Maturity, Due after ten years, Estimated Fair Value	2,085,734
Held to Maturity, Estimated Fair Value	2,203,726	2,236,285
Available for Sale, Due in one year or less, Amortized Cost	240,643
Available for Sale, Due after one year through five years, Amortized Cost	340,268
Available for Sale, Due after five years through ten years, Amortized Cost	661,716
Available for Sale, Due after ten years, Amortized Cost	4,305,677
Debt Securities, Available-for-sale, Amortized Cost	5,548,304	4,842,060
Available for Sale, Due in one year or less, Estimated Fair Value	240,683
Available for Sale, Due after one year through five years, Estimated Fair Value	340,030
Available for Sale, Due after five years through ten years, Estimated Fair Value	636,221
Available for Sale, Due after ten years, Estimated Fair Value	4,132,923
Debt Securities, Available-for-sale	5,349,857	4,560,224
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	1,499
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	1,499	1,497
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	42,428	43,179
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	$ 42,428	$ 43,179