Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 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	$ 3,801,940	$ 4,621,027
Investment securities available for sale, gross unrealized gains	165,147	210,649
Investment securities available for sale, gross unrealized losses	7,304	9,070
Investment securities available for sale, estimated fair value	3,959,783	4,822,606
Amortized cost for held to maturity	1,718,198	1,748,989
Gross unrealized gains for held to maturity	81,715	111,241
Gross unrealized losses for held to maturity	14,899	17,949
Estimated fair value for held to maturity	1,785,014	1,842,281
Equity and other securities, Amortized Cost	463,043	449,008
Equity securities, Gross Unrealized Gains	2,896	25,094
Equity securities, Gross Unrealized Losses	743
Equity and other securities, Estimated Fair Value	465,196	474,102
Other securities, Amortized cost	386,525	381,117
Other securities, Estimated fair value	386,525	381,117
Total debt securities Amortized cost	5,520,138	6,370,016
Total debt securities Gross unrealized gains	246,862	321,890
Total debt securities Gross unrealized losses	22,203	27,019
Total debt securities Estimated fair value	5,744,797	6,664,887
Readily marketable securities Amortized cost	76,518	67,891
Readily marketable securities Gross unrealized gains	2,896	25,094
Readily marketable securities Gross unrealized losses	743
Readily marketable securities Estimated fair value	78,671	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	6,877	9,154
Investment securities available for sale, gross unrealized gains	147	198
Investment securities available for sale, gross unrealized losses	1	14
Investment securities available for sale, estimated fair value	7,023	9,338
Amortized cost for held to maturity	3,080	2,999
Gross unrealized losses for held to maturity	2
Estimated fair value for held to maturity	3,078	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	546	1,531
Gross unrealized gains for held to maturity	3	9
Estimated fair value for held to maturity	549	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	3,658,704	4,475,406
Investment securities available for sale, gross unrealized gains	162,369	208,787
Investment securities available for sale, gross unrealized losses	583	755
Investment securities available for sale, estimated fair value	3,820,490	4,683,438
Amortized cost for held to maturity	1,643,433	1,664,443
Gross unrealized gains for held to maturity	70,560	100,176
Gross unrealized losses for held to maturity	95	11
Estimated fair value for held to maturity	1,713,898	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	136,359	136,451
Investment securities available for sale, gross unrealized gains	2,631	1,664
Investment securities available for sale, gross unrealized losses	6,720	8,301
Investment securities available for sale, estimated fair value	132,270	129,814
Amortized cost for held to maturity	2,665	2,861
Estimated fair value for held to maturity	2,665	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	68,474	77,155
Gross unrealized gains for held to maturity	11,152	11,056
Gross unrealized losses for held to maturity	14,802	17,938
Estimated fair value for held to maturity	$ 64,824	$ 70,273

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