Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2026	Dec. 31, 2025
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 34,412
Held to Maturity, Due after one year through five years, Amortized Cost	499,617
Held to Maturity, Due after five years through ten years, Amortized Cost	140,616
Held to Maturity, Due after ten years, Amortized Cost	2,688,643
Held to Maturity, Amortized Cost	3,363,288	$ 3,431,679
Held to Maturity, Due in one year or less, Estimated Fair Value	34,382
Held to Maturity, Due after one year through five years, Estimated Fair Value	460,061
Held to Maturity, Due after five years through ten years, Estimated Fair Value	126,065
Held to Maturity, Due after ten years, Estimated Fair Value	2,332,270
Held to Maturity, Estimated Fair Value	2,952,778	3,295,431
Available-for-sale Securities, Due in one year or less, Amortized Cost	539,660
Available for Sale, Due after one year through five years, Amortized Cost	2,253,142
Available for Sale, Due after five years through ten years, Amortized Cost	401,051
Available for Sale, Due after ten years, Amortized Cost	15,869,871
Debt Securities, Available-for-sale, Amortized Cost	19,105,977	17,013,723
Available for Sale, Due in one year or less, Estimated Fair Value	538,079
Available for Sale, Due after one year through five years, Estimated Fair Value	2,122,495
Available for Sale, Due after five years through ten years, Estimated Fair Value	353,749
Available for Sale, Due after ten years, Estimated Fair Value	14,042,052
Securities available for sale, at estimated fair value	17,098,628	15,970,596
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	449,045
Available for Sale, Due after one year through five years, Amortized Cost	1,938,321
Available for Sale, Due after five years through ten years, Amortized Cost	0
Available for Sale, Due after ten years, Amortized Cost	177,011
Debt Securities, Available-for-sale, Amortized Cost	2,564,377	2,604,852
Available for Sale, Due in one year or less, Estimated Fair Value	447,555
Available for Sale, Due after one year through five years, Estimated Fair Value	1,818,975
Available for Sale, Due after five years through ten years, Estimated Fair Value	0
Available for Sale, Due after ten years, Estimated Fair Value	116,984
Securities available for sale, at estimated fair value	2,383,514	2,456,517
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	493,164
Held to Maturity, Due after five years through ten years, Amortized Cost	11,203
Held to Maturity, Due after ten years, Amortized Cost	553,708
Held to Maturity, Amortized Cost	1,058,075	1,113,474
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	453,672
Held to Maturity, Due after five years through ten years, Estimated Fair Value	9,101
Held to Maturity, Due after ten years, Estimated Fair Value	520,348
Held to Maturity, Estimated Fair Value	983,121	1,090,206
Available-for-sale Securities, Due in one year or less, Amortized Cost	15
Available for Sale, Due after one year through five years, Amortized Cost	10,029
Available for Sale, Due after five years through ten years, Amortized Cost	1,766
Available for Sale, Due after ten years, Amortized Cost	10,824,689
Debt Securities, Available-for-sale, Amortized Cost	10,836,499	8,818,139
Available for Sale, Due in one year or less, Estimated Fair Value	15
Available for Sale, Due after one year through five years, Estimated Fair Value	10,012
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,744
Available for Sale, Due after ten years, Estimated Fair Value	9,529,295
Securities available for sale, at estimated fair value	9,541,066	8,121,794
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	32,912
Held to Maturity, Due after one year through five years, Amortized Cost	6,453
Held to Maturity, Due after five years through ten years, Amortized Cost	129,413
Held to Maturity, Due after ten years, Amortized Cost	2,134,935
Held to Maturity, Amortized Cost	2,303,713	2,316,705
Held to Maturity, Due in one year or less, Estimated Fair Value	32,884
Held to Maturity, Due after one year through five years, Estimated Fair Value	6,389
Held to Maturity, Due after five years through ten years, Estimated Fair Value	116,964
Held to Maturity, Due after ten years, Estimated Fair Value	1,811,922
Held to Maturity, Estimated Fair Value	1,968,159	2,203,726
Available-for-sale Securities, Due in one year or less, Amortized Cost	90,600
Available for Sale, Due after one year through five years, Amortized Cost	304,792
Available for Sale, Due after five years through ten years, Amortized Cost	399,285
Available for Sale, Due after ten years, Amortized Cost	4,868,171
Debt Securities, Available-for-sale, Amortized Cost	5,662,848	5,548,304
Available for Sale, Due in one year or less, Estimated Fair Value	90,509
Available for Sale, Due after one year through five years, Estimated Fair Value	293,508
Available for Sale, Due after five years through ten years, Estimated Fair Value	352,005
Available for Sale, Due after ten years, Estimated Fair Value	4,395,773
Securities available for sale, at estimated fair value	5,131,795	5,349,857
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	1,498
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	1,498	1,499
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	42,253	42,428
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	$ 42,253	$ 42,428