Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 350,368
Held to Maturity, Due after one year through five years, Amortized Cost	85,553
Held to Maturity, Due after five years through ten years, Amortized Cost	536,684
Held to Maturity, Due after ten years, Amortized Cost	973,981
Held to Maturity, Amortized Cost	1,946,586	$ 1,749,337
Held to Maturity, Due in one year or less, Estimated Fair Value	351,240
Held to Maturity, Due after one year through five years, Estimated Fair Value	86,226
Held to Maturity, Due after five years through ten years, Estimated Fair Value	497,887
Held to Maturity, Due after ten years, Estimated Fair Value	893,162
Held to Maturity, Estimated Fair Value	1,828,515	1,809,143
Available-for-sale Securities, Due in one year or less, Amortized Cost	207,133
Available for Sale, Due after one year through five years, Amortized Cost	4,589,781
Available for Sale, Due after five years through ten years, Amortized Cost	2,355,016
Available for Sale, Due after ten years, Amortized Cost	10,655,968
Debt Securities, Available-for-sale, Amortized Cost	17,850,263	13,445,888
Available for Sale, Due in one year or less, Estimated Fair Value	209,193
Available for Sale, Due after one year through five years, Estimated Fair Value	4,547,346
Available for Sale, Due after five years through ten years, Estimated Fair Value	2,213,463
Available for Sale, Due after ten years, Estimated Fair Value	9,769,318
Securities available for sale, at estimated fair value	16,781,685	13,924,628
US Treasury Securities [Member]
Schedule of Investments [Line Items]
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	3,031,706
Available for Sale, Due after five years through ten years, Amortized Cost	1,433,320
Available for Sale, Due after ten years, Amortized Cost	191,870
Debt Securities, Available-for-sale, Amortized Cost	4,656,896	2,165,702
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	2,957,018
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,301,539
Available for Sale, Due after ten years, Estimated Fair Value	157,742
Securities available for sale, at estimated fair value	4,416,299	2,179,433
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	3
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	514,582
Held to Maturity, Due after ten years, Amortized Cost	12,090
Held to Maturity, Amortized Cost	526,675	527,264
Held to Maturity, Due in one year or less, Estimated Fair Value	3
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	475,721
Held to Maturity, Due after ten years, Estimated Fair Value	10,568
Held to Maturity, Estimated Fair Value	486,292	546,030
Available-for-sale Securities, Due in one year or less, Amortized Cost	55
Available for Sale, Due after one year through five years, Amortized Cost	11,080
Available for Sale, Due after five years through ten years, Amortized Cost	18,743
Available for Sale, Due after ten years, Amortized Cost	5,937,925
Debt Securities, Available-for-sale, Amortized Cost	5,967,803	4,059,692
Available for Sale, Due in one year or less, Estimated Fair Value	55
Available for Sale, Due after one year through five years, Estimated Fair Value	11,231
Available for Sale, Due after five years through ten years, Estimated Fair Value	18,792
Available for Sale, Due after ten years, Estimated Fair Value	5,305,647
Securities available for sale, at estimated fair value	5,335,725	4,066,265
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	350,365
Held to Maturity, Due after one year through five years, Amortized Cost	84,053
Held to Maturity, Due after five years through ten years, Amortized Cost	22,102
Held to Maturity, Due after ten years, Amortized Cost	961,891
Held to Maturity, Amortized Cost	1,418,411	1,220,573
Held to Maturity, Due in one year or less, Estimated Fair Value	351,237
Held to Maturity, Due after one year through five years, Estimated Fair Value	84,787
Held to Maturity, Due after five years through ten years, Estimated Fair Value	22,166
Held to Maturity, Due after ten years, Estimated Fair Value	882,594
Held to Maturity, Estimated Fair Value	1,340,784	1,261,613
Available-for-sale Securities, Due in one year or less, Amortized Cost	207,078
Available for Sale, Due after one year through five years, Amortized Cost	1,546,995
Available for Sale, Due after five years through ten years, Amortized Cost	902,953
Available for Sale, Due after ten years, Amortized Cost	4,526,173
Debt Securities, Available-for-sale, Amortized Cost	7,183,199	7,178,135
Available for Sale, Due in one year or less, Estimated Fair Value	209,138
Available for Sale, Due after one year through five years, Estimated Fair Value	1,579,097
Available for Sale, Due after five years through ten years, Estimated Fair Value	893,132
Available for Sale, Due after ten years, Estimated Fair Value	4,305,929
Securities available for sale, at estimated fair value	6,987,296	7,636,571
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	1,439
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	1,439	1,500
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	42,365	42,359
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	$ 42,365	$ 42,359