Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 3,473,923	$ 4,621,027
Investment securities available for sale, gross unrealized gains	150,971	210,649
Investment securities available for sale, gross unrealized losses	6,788	9,070
Investment securities available for sale, estimated fair value	3,618,106	4,822,606
Amortized cost for held to maturity	2,359,727	1,748,989
Gross unrealized gains for held to maturity	74,424	111,241
Gross unrealized losses for held to maturity	20,213	17,949
Estimated fair value for held to maturity	2,413,938	1,842,281
Equity and other securities, Amortized Cost	467,349	449,008
Equity securities, Gross Unrealized Gains	3,341	25,094
Equity securities, Gross Unrealized Losses	901
Equity and other securities, Estimated Fair Value	469,789	474,102
Other securities, Amortized cost	387,333	381,117
Other securities, Estimated fair value	387,333	381,117
Total debt securities Amortized cost	5,833,650	6,370,016
Total debt securities Gross unrealized gains	225,395	321,890
Total debt securities Gross unrealized losses	27,001	27,019
Total debt securities Estimated fair value	6,032,044	6,664,887
Readily marketable securities Amortized cost	80,016	67,891
Readily marketable securities Gross unrealized gains	3,341	25,094
Readily marketable securities Gross unrealized losses	901
Readily marketable securities Estimated fair value	82,456	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	9,621	9,154
Investment securities available for sale, gross unrealized gains	124	198
Investment securities available for sale, gross unrealized losses	1	14
Investment securities available for sale, estimated fair value	9,744	9,338
Amortized cost for held to maturity	3,065	2,999
Estimated fair value for held to maturity	3,065	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		9
Estimated fair value for held to maturity	177	1,540
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	3,328,080	4,475,406
Investment securities available for sale, gross unrealized gains	147,942	208,787
Investment securities available for sale, gross unrealized losses	543	755
Investment securities available for sale, estimated fair value	3,475,479	4,683,438
Amortized cost for held to maturity	2,288,934	1,664,443
Gross unrealized gains for held to maturity	63,523	100,176
Gross unrealized losses for held to maturity	6,312	11
Estimated fair value for held to maturity	2,346,145	1,764,608
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	136,222	136,451
Investment securities available for sale, gross unrealized gains	2,905	1,664
Investment securities available for sale, gross unrealized losses	6,244	8,301
Investment securities available for sale, estimated fair value	132,883	129,814
Amortized cost for held to maturity	2,622	2,861
Estimated fair value for held to maturity	2,622	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	64,929	77,155
Gross unrealized gains for held to maturity	10,901	11,056
Gross unrealized losses for held to maturity	13,901	17,938
Estimated fair value for held to maturity	$ 61,929	$ 70,273

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