Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 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	$ 8,879,416	$ 3,849,347
Investment securities available for sale, gross unrealized gains	4,080	115,305
Investment securities available for sale, gross unrealized losses	178,541	8,848
Investment securities available for sale, estimated fair value	8,704,955	3,955,804
Amortized cost for held to maturity	13,373,171	2,734,674
Gross unrealized gains for held to maturity	10,504	59,743
Gross unrealized losses for held to maturity	687,560	23,127
Estimated fair value for held to maturity	12,696,115	2,771,290
Equity and other securities, Amortized Cost	720,530	461,516
Equity securities, Gross Unrealized Gains	5,189	4,460
Equity securities, Gross Unrealized Losses	(2,128)	594
Equity and other securities, Estimated Fair Value	723,591	465,382
Other securities, Amortized cost	621,719	387,742
Other securities, Estimated fair value	621,719	387,742
Total debt securities Amortized cost	22,252,587	6,584,021
Total debt securities Gross unrealized gains	14,584	175,048
Total debt securities Gross unrealized losses	866,101	31,975
Total debt securities Estimated fair value	21,401,070	6,727,094
Readily marketable securities Amortized cost	98,811	73,774
Readily marketable securities Gross unrealized gains	5,189	4,460
Readily marketable securities Gross unrealized losses	(2,128)	594
Readily marketable securities Estimated fair value	101,872	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	5,273,377	682,267
Investment securities available for sale, gross unrealized gains	1,568	229
Investment securities available for sale, gross unrealized losses	92,374	3,806
Investment securities available for sale, estimated fair value	5,182,571	678,690
Amortized cost for held to maturity	1,337,395	3,052
Gross unrealized losses for held to maturity	18,339	9
Estimated fair value for held to maturity	1,319,056	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	2,735,757	177
Gross unrealized gains for held to maturity	53	2
Gross unrealized losses for held to maturity	108,197
Estimated fair value for held to maturity	2,627,613	179
Commercial [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	617,359
Investment securities available for sale, gross unrealized losses	2,575
Investment securities available for sale, estimated fair value	614,784
Amortized cost for held to maturity	934,161
Gross unrealized losses for held to maturity	47,527
Estimated fair value for held to maturity	886,634
Residential [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	2,799,742	3,042,771
Investment securities available for sale, gross unrealized gains	2,079	113,102
Investment securities available for sale, gross unrealized losses	75,052	561
Investment securities available for sale, estimated fair value	2,726,769	3,155,312
Amortized cost for held to maturity	8,308,155	2,667,328
Gross unrealized gains for held to maturity	116	49,221
Gross unrealized losses for held to maturity	503,293	8,376
Estimated fair value for held to maturity	7,804,978	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	188,938	124,309
Investment securities available for sale, gross unrealized gains	433	1,974
Investment securities available for sale, gross unrealized losses	8,540	4,481
Investment securities available for sale, estimated fair value	180,831	121,802
Amortized cost for held to maturity	2,306	2,562
Estimated fair value for held to maturity	2,306	2,562
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	55,397	61,555
Gross unrealized gains for held to maturity	10,335	10,520
Gross unrealized losses for held to maturity	10,204	14,742
Estimated fair value for held to maturity	$ 55,528	$ 57,333

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