Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 11,340,155	$ 3,849,347
Investment securities available for sale, gross unrealized gains	851	115,305
Investment securities available for sale, gross unrealized losses	470,660	8,848
Investment securities available for sale, estimated fair value	10,870,346	3,955,804
Amortized cost for held to maturity	12,898,862	2,734,674
Gross unrealized gains for held to maturity	9,266	59,743
Gross unrealized losses for held to maturity	1,353,473	23,127
Estimated fair value for held to maturity	11,554,655	2,771,290
Equity and other securities, Amortized Cost	832,604	461,516
Equity securities, Gross Unrealized Gains	4,943	4,460
Equity securities, Gross Unrealized Losses	(2,990)	594
Equity and other securities, Estimated Fair Value	834,557	465,382
Other securities, Amortized cost	640,020	387,742
Other securities, Estimated fair value	640,020	387,742
Total debt securities Amortized cost	24,239,017	6,584,021
Total debt securities Gross unrealized gains	10,117	175,048
Total debt securities Gross unrealized losses	1,824,133	31,975
Total debt securities Estimated fair value	22,425,001	6,727,094
Readily marketable securities Amortized cost	192,584	73,774
Readily marketable securities Gross unrealized gains	4,943	4,460
Readily marketable securities Gross unrealized losses	2,990	594
Readily marketable securities Estimated fair value	194,537	77,640
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	7,908,153	682,267
Investment securities available for sale, gross unrealized gains	413	229
Investment securities available for sale, gross unrealized losses	249,838	3,806
Investment securities available for sale, estimated fair value	7,658,728	678,690
Amortized cost for held to maturity	1,199,539	3,052
Gross unrealized losses for held to maturity	49,633	9
Estimated fair value for held to maturity	1,149,906	3,043
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	2,663,917	177
Gross unrealized gains for held to maturity	6	2
Gross unrealized losses for held to maturity	221,631
Estimated fair value for held to maturity	2,442,292	179
Commercial [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	613,752
Investment securities available for sale, gross unrealized losses	7,181
Investment securities available for sale, estimated fair value	606,571
Amortized cost for held to maturity	927,503
Gross unrealized losses for held to maturity	106,215
Estimated fair value for held to maturity	821,288
Residential [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	2,635,976	3,042,771
Investment securities available for sale, gross unrealized gains	127	113,102
Investment securities available for sale, gross unrealized losses	205,157	561
Investment securities available for sale, estimated fair value	2,430,946	3,155,312
Amortized cost for held to maturity	8,053,619	2,667,328
Gross unrealized gains for held to maturity		49,221
Gross unrealized losses for held to maturity	967,653	8,376
Estimated fair value for held to maturity	7,085,966	2,708,173
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	182,274	124,309
Investment securities available for sale, gross unrealized gains	311	1,974
Investment securities available for sale, gross unrealized losses	8,484	4,481
Investment securities available for sale, estimated fair value	174,101	121,802
Amortized cost for held to maturity	1,805	2,562
Estimated fair value for held to maturity	1,805	2,562
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	52,479	61,555
Gross unrealized gains for held to maturity	9,260	10,520
Gross unrealized losses for held to maturity	8,341	14,742
Estimated fair value for held to maturity	$ 53,398	$ 57,333

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