Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 25,402
Held to Maturity, Due after one year through five years, Amortized Cost	500,114
Held to Maturity, Due after five years through ten years, Amortized Cost	128,577
Held to Maturity, Due after ten years, Amortized Cost	2,731,914
Held to Maturity, Amortized Cost	3,386,007	$ 3,431,679
Held to Maturity, Due in one year or less, Estimated Fair Value	25,502
Held to Maturity, Due after one year through five years, Estimated Fair Value	468,912
Held to Maturity, Due after five years through ten years, Estimated Fair Value	125,604
Held to Maturity, Due after ten years, Estimated Fair Value	2,630,798
Held to Maturity, Estimated Fair Value	3,250,816	3,295,431
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,557,289
Available for Sale, Due after one year through five years, Amortized Cost	1,771,318
Available for Sale, Due after five years through ten years, Amortized Cost	473,893
Available for Sale, Due after ten years, Amortized Cost	15,299,474
Debt Securities, Available-for-sale, Amortized Cost	19,144,541	17,013,723
Available for Sale, Due in one year or less, Estimated Fair Value	1,554,014
Available for Sale, Due after one year through five years, Estimated Fair Value	1,673,783
Available for Sale, Due after five years through ten years, Estimated Fair Value	443,195
Available for Sale, Due after ten years, Estimated Fair Value	14,376,414
Securities available for sale, at estimated fair value	18,089,973	15,970,596
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,443,787
Available for Sale, Due after one year through five years, Amortized Cost	1,443,038
Available for Sale, Due after five years through ten years, Amortized Cost	0
Available for Sale, Due after ten years, Amortized Cost	193,355
Debt Securities, Available-for-sale, Amortized Cost	3,080,180	2,604,852
Available for Sale, Due in one year or less, Estimated Fair Value	1,440,481
Available for Sale, Due after one year through five years, Estimated Fair Value	1,346,189
Available for Sale, Due after five years through ten years, Estimated Fair Value	0
Available for Sale, Due after ten years, Estimated Fair Value	139,281
Securities available for sale, at estimated fair value	2,925,951	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	493,666
Held to Maturity, Due after five years through ten years, Amortized Cost	11,209
Held to Maturity, Due after ten years, Amortized Cost	573,647
Held to Maturity, Amortized Cost	1,078,522	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	462,353
Held to Maturity, Due after five years through ten years, Estimated Fair Value	9,647
Held to Maturity, Due after ten years, Estimated Fair Value	572,003
Held to Maturity, Estimated Fair Value	1,044,003	1,090,206
Available-for-sale Securities, Due in one year or less, Amortized Cost	27
Available for Sale, Due after one year through five years, Amortized Cost	10,092
Available for Sale, Due after five years through ten years, Amortized Cost	1,846
Available for Sale, Due after ten years, Amortized Cost	10,546,979
Debt Securities, Available-for-sale, Amortized Cost	10,558,944	8,818,139
Available for Sale, Due in one year or less, Estimated Fair Value	27
Available for Sale, Due after one year through five years, Estimated Fair Value	10,085
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,865
Available for Sale, Due after ten years, Estimated Fair Value	9,807,333
Securities available for sale, at estimated fair value	9,819,310	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	23,902
Held to Maturity, Due after one year through five years, Amortized Cost	6,448
Held to Maturity, Due after five years through ten years, Amortized Cost	117,368
Held to Maturity, Due after ten years, Amortized Cost	2,158,267
Held to Maturity, Amortized Cost	2,305,985	2,316,705
Held to Maturity, Due in one year or less, Estimated Fair Value	24,002
Held to Maturity, Due after one year through five years, Estimated Fair Value	6,559
Held to Maturity, Due after five years through ten years, Estimated Fair Value	115,957
Held to Maturity, Due after ten years, Estimated Fair Value	2,058,795
Held to Maturity, Estimated Fair Value	2,205,313	2,203,726
Available-for-sale Securities, Due in one year or less, Amortized Cost	113,475
Available for Sale, Due after one year through five years, Amortized Cost	318,188
Available for Sale, Due after five years through ten years, Amortized Cost	472,047
Available for Sale, Due after ten years, Amortized Cost	4,559,140
Debt Securities, Available-for-sale, Amortized Cost	5,462,850	5,548,304
Available for Sale, Due in one year or less, Estimated Fair Value	113,506
Available for Sale, Due after one year through five years, Estimated Fair Value	317,509
Available for Sale, Due after five years through ten years, Estimated Fair Value	441,330
Available for Sale, Due after ten years, Estimated Fair Value	4,429,800
Securities available for sale, at estimated fair value	5,302,145	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	1,500
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	1,500	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	42,567	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	$ 42,567	$ 42,428