Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 11,038,966	$ 11,193,152
Investment securities available for sale, gross unrealized gains	154	515
Investment securities available for sale, gross unrealized losses	446,711	444,706
Investment securities available for sale, estimated fair value	10,592,409	10,748,961
Amortized cost for held to maturity	15,571,120	13,529,969
Gross unrealized gains for held to maturity	9,041	10,288
Gross unrealized losses for held to maturity	1,815,218	1,164,837
Estimated fair value for held to maturity	13,764,943	12,375,420
Equity and other securities, Amortized Cost	1,174,046	933,766
Equity securities, Gross Unrealized Gains	2,425	2,120
Equity securities, Gross Unrealized Losses	3,897	3,945
Equity and other securities, Estimated Fair Value	1,172,574	931,941
Other securities, Amortized cost	913,358	780,483
Other securities, Estimated fair value	913,358	780,483
Total debt securities Amortized cost	26,610,086	24,723,121
Total debt securities Gross unrealized gains	9,195	10,803
Total debt securities Gross unrealized losses	2,261,929	1,609,543
Total debt securities Estimated fair value	24,357,352	23,124,381
Readily marketable securities Amortized cost	260,688	153,283
Readily marketable securities Gross unrealized gains	2,425	2,120
Readily marketable securities Gross unrealized losses	3,897	3,945
Readily marketable securities Estimated fair value	259,216	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	1,001,991	7,913,932
Investment securities available for sale, gross unrealized gains		200
Investment securities available for sale, gross unrealized losses	48,706	243,172
Investment securities available for sale, estimated fair value	953,285	7,670,960
Amortized cost for held to maturity	7,935,658	1,054,035
Gross unrealized losses for held to maturity	200,551	45,747
Estimated fair value for held to maturity	7,735,107	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	2,511,784	2,577,078
Gross unrealized gains for held to maturity		4
Gross unrealized losses for held to maturity	219,149	116,512
Estimated fair value for held to maturity	2,292,635	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	544,263	594,779
Investment securities available for sale, gross unrealized losses	18,794	20,480
Investment securities available for sale, estimated fair value	525,469	574,299
Amortized cost for held to maturity	2,031,532	912,431
Gross unrealized losses for held to maturity	233,055	103,528
Estimated fair value for held to maturity	1,798,477	808,903
Residential [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	2,382,810	2,501,334
Investment securities available for sale, gross unrealized gains	8	65
Investment securities available for sale, gross unrealized losses	214,531	171,281
Investment securities available for sale, estimated fair value	2,168,287	2,330,118
Amortized cost for held to maturity	9,979,882	8,934,918
Gross unrealized gains for held to maturity		1,451
Gross unrealized losses for held to maturity	1,306,934	891,063
Estimated fair value for held to maturity	8,672,948	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	176,235	183,107
Investment securities available for sale, gross unrealized gains	146	250
Investment securities available for sale, gross unrealized losses	12,835	9,773
Investment securities available for sale, estimated fair value	163,546	173,584
Amortized cost for held to maturity	1,612	1,765
Estimated fair value for held to maturity	1,612	1,765
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	44,319	49,742
Gross unrealized gains for held to maturity	9,041	8,833
Gross unrealized losses for held to maturity	7,374	7,987
Estimated fair value for held to maturity	$ 45,986	$ 50,588

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