Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 25,385
Held to Maturity, Due after one year through five years, Amortized Cost	500,608
Held to Maturity, Due after five years through ten years, Amortized Cost	115,290
Held to Maturity, Due after ten years, Amortized Cost	2,764,717
Held to Maturity, Amortized Cost	3,406,000	$ 3,431,679
Held to Maturity, Due in one year or less, Estimated Fair Value	25,534
Held to Maturity, Due after one year through five years, Estimated Fair Value	471,242
Held to Maturity, Due after five years through ten years, Estimated Fair Value	110,595
Held to Maturity, Due after ten years, Estimated Fair Value	2,610,096
Held to Maturity, Estimated Fair Value	3,217,467	3,295,431
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,766,711
Available for Sale, Due after one year through five years, Amortized Cost	1,898,596
Available for Sale, Due after five years through ten years, Amortized Cost	552,786
Available for Sale, Due after ten years, Amortized Cost	13,675,322
Debt Securities, Available-for-sale, Amortized Cost	17,935,916	17,013,723
Available for Sale, Due in one year or less, Estimated Fair Value	1,762,400
Available for Sale, Due after one year through five years, Estimated Fair Value	1,802,685
Available for Sale, Due after five years through ten years, Estimated Fair Value	522,059
Available for Sale, Due after ten years, Estimated Fair Value	12,660,961
Securities available for sale, at estimated fair value	16,790,606	15,970,596
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,685,183
Available for Sale, Due after one year through five years, Amortized Cost	1,566,540
Available for Sale, Due after five years through ten years, Amortized Cost	0
Available for Sale, Due after ten years, Amortized Cost	193,258
Debt Securities, Available-for-sale, Amortized Cost	3,444,981	2,604,852
Available for Sale, Due in one year or less, Estimated Fair Value	1,680,776
Available for Sale, Due after one year through five years, Estimated Fair Value	1,471,989
Available for Sale, Due after five years through ten years, Estimated Fair Value	0
Available for Sale, Due after ten years, Estimated Fair Value	138,789
Securities available for sale, at estimated fair value	3,291,554	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	494,165
Held to Maturity, Due after five years through ten years, Amortized Cost	11,215
Held to Maturity, Due after ten years, Amortized Cost	592,331
Held to Maturity, Amortized Cost	1,097,711	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	464,693
Held to Maturity, Due after five years through ten years, Estimated Fair Value	9,668
Held to Maturity, Due after ten years, Estimated Fair Value	592,092
Held to Maturity, Estimated Fair Value	1,066,453	1,090,206
Available-for-sale Securities, Due in one year or less, Amortized Cost	40
Available for Sale, Due after one year through five years, Amortized Cost	9,770
Available for Sale, Due after five years through ten years, Amortized Cost	2,364
Available for Sale, Due after ten years, Amortized Cost	9,008,903
Debt Securities, Available-for-sale, Amortized Cost	9,021,077	8,818,139
Available for Sale, Due in one year or less, Estimated Fair Value	39
Available for Sale, Due after one year through five years, Estimated Fair Value	9,748
Available for Sale, Due after five years through ten years, Estimated Fair Value	2,408
Available for Sale, Due after ten years, Estimated Fair Value	8,268,036
Securities available for sale, at estimated fair value	8,280,231	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	23,885
Held to Maturity, Due after one year through five years, Amortized Cost	6,443
Held to Maturity, Due after five years through ten years, Amortized Cost	104,075
Held to Maturity, Due after ten years, Amortized Cost	2,172,386
Held to Maturity, Amortized Cost	2,306,789	2,316,705
Held to Maturity, Due in one year or less, Estimated Fair Value	24,036
Held to Maturity, Due after one year through five years, Estimated Fair Value	6,549
Held to Maturity, Due after five years through ten years, Estimated Fair Value	100,927
Held to Maturity, Due after ten years, Estimated Fair Value	2,018,004
Held to Maturity, Estimated Fair Value	2,149,516	2,203,726
Available-for-sale Securities, Due in one year or less, Amortized Cost	81,488
Available for Sale, Due after one year through five years, Amortized Cost	322,286
Available for Sale, Due after five years through ten years, Amortized Cost	550,422
Available for Sale, Due after ten years, Amortized Cost	4,473,161
Debt Securities, Available-for-sale, Amortized Cost	5,427,357	5,548,304
Available for Sale, Due in one year or less, Estimated Fair Value	81,585
Available for Sale, Due after one year through five years, Estimated Fair Value	320,948
Available for Sale, Due after five years through ten years, Estimated Fair Value	519,651
Available for Sale, Due after ten years, Estimated Fair Value	4,254,136
Securities available for sale, at estimated fair value	5,176,320	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	42,501	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	$ 42,501	$ 42,428