Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 11,393,916	$ 11,193,152
Investment securities available for sale, gross unrealized gains	3,070	515
Investment securities available for sale, gross unrealized losses	358,064	444,706
Investment securities available for sale, estimated fair value	11,038,922	10,748,961
Amortized cost for held to maturity	16,201,571	13,529,969
Gross unrealized gains for held to maturity	20,949	10,288
Gross unrealized losses for held to maturity	1,018,059	1,164,837
Estimated fair value for held to maturity	15,204,461	12,375,420
Equity and other securities, Amortized Cost	1,204,957	933,766
Equity securities, Gross Unrealized Gains	1,821	2,120
Equity securities, Gross Unrealized Losses	4,062	3,945
Equity and other securities, Estimated Fair Value	1,202,716	931,941
Other securities, Amortized cost	907,496	780,483
Other securities, Estimated fair value	907,496	780,483
Total debt securities Amortized cost	27,595,487	24,723,121
Total debt securities Gross unrealized gains	24,019	10,803
Total debt securities Gross unrealized losses	1,376,123	1,609,543
Total debt securities Estimated fair value	26,243,383	23,124,381
Readily marketable securities Amortized cost	297,461	153,283
Readily marketable securities Gross unrealized gains	1,821	2,120
Readily marketable securities Gross unrealized losses	4,062	3,945
Readily marketable securities Estimated fair value	295,220	151,458
U.S. Treasury and Federal Agencies [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	7,918,648	7,913,932
Investment securities available for sale, gross unrealized gains	1,378	200
Investment securities available for sale, gross unrealized losses	187,327	243,172
Investment securities available for sale, estimated fair value	7,732,699	7,670,960
Amortized cost for held to maturity	1,056,675	1,054,035
Gross unrealized losses for held to maturity	35,427	45,747
Estimated fair value for held to maturity	1,021,248,000	1,008,288
Obligations of States and Political Subdivisions [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	2,564,095	2,577,078
Gross unrealized gains for held to maturity	36	4
Gross unrealized losses for held to maturity	72,224	116,512
Estimated fair value for held to maturity	2,491,907	2,460,570
Commercial [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	664,788	594,779
Investment securities available for sale, gross unrealized gains	465
Investment securities available for sale, gross unrealized losses	17,059	20,480
Investment securities available for sale, estimated fair value	648,194	574,299
Amortized cost for held to maturity	2,027,911	912,431
Gross unrealized losses for held to maturity	106,970	103,528
Estimated fair value for held to maturity	1,920,941	808,903
Residential [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	2,634,142	2,501,334
Investment securities available for sale, gross unrealized gains	1,011	65
Investment securities available for sale, gross unrealized losses	141,448	171,281
Investment securities available for sale, estimated fair value	2,493,705	2,330,118
Amortized cost for held to maturity	10,503,208	8,934,918
Gross unrealized gains for held to maturity	11,462	1,451
Gross unrealized losses for held to maturity	796,479	891,063
Estimated fair value for held to maturity	9,718,191	8,045,306
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	176,338	183,107
Investment securities available for sale, gross unrealized gains	216	250
Investment securities available for sale, gross unrealized losses	12,230	9,773
Investment securities available for sale, estimated fair value	164,324	173,584
Amortized cost for held to maturity	1,726	1,765
Estimated fair value for held to maturity	1,726	1,765
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	47,956	49,742
Gross unrealized gains for held to maturity	9,451	8,833
Gross unrealized losses for held to maturity	6,959	7,987
Estimated fair value for held to maturity	$ 50,448	$ 50,588

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