Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 7,867
Held to Maturity, Due after one year through five years, Amortized Cost	13,552
Held to Maturity, Due after five years through ten years, Amortized Cost	565,474
Held to Maturity, Due after ten years, Amortized Cost	3,041,540
Held to Maturity, Amortized Cost	3,628,433	$ 2,639,241
Held to Maturity, Due in one year or less, Estimated Fair Value	7,863
Held to Maturity, Due after one year through five years, Estimated Fair Value	13,317
Held to Maturity, Due after five years through ten years, Estimated Fair Value	486,858
Held to Maturity, Due after ten years, Estimated Fair Value	2,723,653
Held to Maturity, Estimated Fair Value	3,231,691	2,467,865
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,535,544
Available for Sale, Due after one year through five years, Amortized Cost	2,986,590
Available for Sale, Due after five years through ten years, Amortized Cost	2,064,052
Available for Sale, Due after ten years, Amortized Cost	11,700,161
Debt Securities, Available-for-sale, Amortized Cost	18,329,017	19,908,432
Available for Sale, Due in one year or less, Estimated Fair Value	1,517,626
Available for Sale, Due after one year through five years, Estimated Fair Value	2,821,527
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,828,956
Available for Sale, Due after ten years, Estimated Fair Value	9,906,310
Securities available for sale, at estimated fair value	16,117,089	18,243,605
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	1,285,709
Available for Sale, Due after one year through five years, Amortized Cost	2,794,385
Available for Sale, Due after five years through ten years, Amortized Cost	1,187,343
Available for Sale, Due after ten years, Amortized Cost	192,321
Debt Securities, Available-for-sale, Amortized Cost	5,459,758	5,450,546
Available for Sale, Due in one year or less, Estimated Fair Value	1,269,227
Available for Sale, Due after one year through five years, Estimated Fair Value	2,633,956
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,011,055
Available for Sale, Due after ten years, Estimated Fair Value	128,914
Securities available for sale, at estimated fair value	5,043,152	5,051,587
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	0
Held to Maturity, Due after five years through ten years, Amortized Cost	513,264
Held to Maturity, Due after ten years, Amortized Cost	744,891
Held to Maturity, Amortized Cost	1,258,155	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	439,829
Held to Maturity, Due after ten years, Estimated Fair Value	705,184
Held to Maturity, Estimated Fair Value	1,145,013	460,800
Available-for-sale Securities, Due in one year or less, Amortized Cost	772
Available for Sale, Due after one year through five years, Amortized Cost	3,593
Available for Sale, Due after five years through ten years, Amortized Cost	15,131
Available for Sale, Due after ten years, Amortized Cost	7,569,908
Debt Securities, Available-for-sale, Amortized Cost	7,589,404	7,316,824
Available for Sale, Due in one year or less, Estimated Fair Value	739
Available for Sale, Due after one year through five years, Estimated Fair Value	3,486
Available for Sale, Due after five years through ten years, Estimated Fair Value	14,979
Available for Sale, Due after ten years, Estimated Fair Value	6,287,430
Securities available for sale, at estimated fair value	6,306,634	6,376,236
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	7,867
Held to Maturity, Due after one year through five years, Amortized Cost	12,052
Held to Maturity, Due after five years through ten years, Amortized Cost	52,210
Held to Maturity, Due after ten years, Amortized Cost	2,296,649
Held to Maturity, Amortized Cost	2,368,778	2,111,619
Held to Maturity, Due in one year or less, Estimated Fair Value	7,863
Held to Maturity, Due after one year through five years, Estimated Fair Value	11,880
Held to Maturity, Due after five years through ten years, Estimated Fair Value	47,029
Held to Maturity, Due after ten years, Estimated Fair Value	2,018,469
Held to Maturity, Estimated Fair Value	2,085,241	2,005,634
Available-for-sale Securities, Due in one year or less, Amortized Cost	249,063
Available for Sale, Due after one year through five years, Amortized Cost	188,612
Available for Sale, Due after five years through ten years, Amortized Cost	861,578
Available for Sale, Due after ten years, Amortized Cost	3,937,932
Debt Securities, Available-for-sale, Amortized Cost	5,237,185	7,098,635
Available for Sale, Due in one year or less, Estimated Fair Value	247,660
Available for Sale, Due after one year through five years, Estimated Fair Value	184,085
Available for Sale, Due after five years through ten years, Estimated Fair Value	802,922
Available for Sale, Due after ten years, Estimated Fair Value	3,489,966
Securities available for sale, at estimated fair value	4,724,633	6,773,355
Other Investments [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	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	1,437
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	1,437	1,431
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	42,670	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
Securities available for sale, at estimated fair value	$ 42,670	$ 42,427