Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 4,621,027	$ 6,258,276
Investment securities available for sale, gross unrealized gains	210,649	90,723
Investment securities available for sale, gross unrealized losses	9,070	30,223
Investment securities available for sale, estimated fair value	4,822,606	6,318,776
Amortized cost for held to maturity	1,748,989	2,656,917
Gross unrealized gains for held to maturity	111,241	62,462
Gross unrealized losses for held to maturity	17,949	20,173
Estimated fair value for held to maturity	1,842,281	2,699,206
Equity and other securities, Amortized Cost	449,008	487,041
Equity securities, Gross Unrealized Gains	25,094	34,786
Equity securities, Gross Unrealized Losses		269
Equity and other securities, Estimated Fair Value	474,102	521,558
Other securities, Amortized cost	381,117	381,517
Other securities, Estimated fair value	381,117	381,517
Total debt securities Amortized cost	6,370,016	8,915,193
Total debt securities Gross unrealized gains	321,890	153,185
Total debt securities Gross unrealized losses	27,019	50,396
Total debt securities Estimated fair value	6,664,887	9,017,982
Readily marketable securities Amortized cost	67,891	105,524
Readily marketable securities Gross unrealized gains	25,094	34,786
Readily marketable securities Gross unrealized losses		269
Readily marketable securities Estimated fair value	92,985	140,041
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	9,154	9,742
Investment securities available for sale, gross unrealized gains	198	41
Investment securities available for sale, gross unrealized losses	14	16
Investment securities available for sale, estimated fair value	9,338	9,767
Amortized cost for held to maturity	2,999	249,862
Gross unrealized gains for held to maturity		286
Estimated fair value for held to maturity	2,999	250,148
Obligations of States and Political Subdivisions [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost		776
Investment securities available for sale, gross unrealized gains		2
Investment securities available for sale, gross unrealized losses		3
Investment securities available for sale, estimated fair value		775
Amortized cost for held to maturity	1,531	4,140
Gross unrealized gains for held to maturity	9	16
Estimated fair value for held to maturity	1,540	4,156
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	4,475,406	6,113,913
Investment securities available for sale, gross unrealized gains	208,787	88,634
Investment securities available for sale, gross unrealized losses	755	21,607
Investment securities available for sale, estimated fair value	4,683,438	6,180,940
Amortized cost for held to maturity	1,664,443	2,306,180
Gross unrealized gains for held to maturity	100,176	50,381
Gross unrealized losses for held to maturity	11	1,992
Estimated fair value for held to maturity	1,764,608	2,354,569
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	16	16
Investment securities available for sale, estimated fair value	16	16
Amortized cost for held to maturity	77,155	93,496
Gross unrealized gains for held to maturity	11,056	11,779
Gross unrealized losses for held to maturity	17,938	18,181
Estimated fair value for held to maturity	70,273	87,094
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	136,451	133,829
Investment securities available for sale, gross unrealized gains	1,664	2,046
Investment securities available for sale, gross unrealized losses	8,301	8,597
Investment securities available for sale, estimated fair value	129,814	127,278
Amortized cost for held to maturity	2,861	3,239
Estimated fair value for held to maturity	$ 2,861	$ 3,239

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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