Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 3,849,347	$ 4,621,027
Investment securities available for sale, gross unrealized gains	115,305	210,649
Investment securities available for sale, gross unrealized losses	8,848	9,070
Investment securities available for sale, estimated fair value	3,955,804	4,822,606
Amortized cost for held to maturity	2,734,674	1,748,989
Gross unrealized gains for held to maturity	59,743	111,241
Gross unrealized losses for held to maturity	23,127	17,949
Estimated fair value for held to maturity	2,771,290	1,842,281
Equity and other securities, Amortized Cost	461,516	449,008
Equity securities, Gross Unrealized Gains	4,460	25,094
Equity securities, Gross Unrealized Losses	594
Equity and other securities, Estimated Fair Value	465,382	474,102
Other securities, Amortized cost	387,742	381,117
Other securities, Estimated fair value	387,742	381,117
Total debt securities Amortized cost	6,584,021	6,370,016
Total debt securities Gross unrealized gains	175,048	321,890
Total debt securities Gross unrealized losses	31,975	27,019
Total debt securities Estimated fair value	6,727,094	6,664,887
Readily marketable securities Amortized cost	73,774	67,891
Readily marketable securities Gross unrealized gains	4,460	25,094
Readily marketable securities Gross unrealized losses	594
Readily marketable securities Estimated fair value	77,640	92,985
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	682,267	9,154
Investment securities available for sale, gross unrealized gains	229	198
Investment securities available for sale, gross unrealized losses	3,806	14
Investment securities available for sale, estimated fair value	678,690	9,338
Amortized cost for held to maturity	3,052	2,999
Gross unrealized losses for held to maturity	9
Estimated fair value for held to maturity	3,043	2,999
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	177	1,531
Gross unrealized gains for held to maturity	2	9
Estimated fair value for held to maturity	179	1,540
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	124,309	136,451
Investment securities available for sale, gross unrealized gains	1,974	1,664
Investment securities available for sale, gross unrealized losses	4,481	8,301
Investment securities available for sale, estimated fair value	121,802	129,814
Amortized cost for held to maturity	2,562	2,861
Estimated fair value for held to maturity	2,562	2,861
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost		16
Investment securities available for sale, estimated fair value		16
Amortized cost for held to maturity	61,555	77,155
Gross unrealized gains for held to maturity	10,520	11,056
Gross unrealized losses for held to maturity	14,742	17,938
Estimated fair value for held to maturity	57,333	70,273
Government Issued or Guaranteed [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	3,042,771	4,475,406
Investment securities available for sale, gross unrealized gains	113,102	208,787
Investment securities available for sale, gross unrealized losses	561	755
Investment securities available for sale, estimated fair value	3,155,312	4,683,438
Amortized cost for held to maturity	2,667,328	1,664,443
Gross unrealized gains for held to maturity	49,221	100,176
Gross unrealized losses for held to maturity	8,376	11
Estimated fair value for held to maturity	$ 2,708,173	$ 1,764,608

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