Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 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	$ 4,194,886	$ 4,621,027
Investment securities available for sale, gross unrealized gains	176,612	210,649
Investment securities available for sale, gross unrealized losses	7,061	9,070
Investment securities available for sale, estimated fair value	4,364,437	4,822,606
Amortized cost for held to maturity	1,778,135	1,748,989
Gross unrealized gains for held to maturity	90,942	111,241
Gross unrealized losses for held to maturity	18,867	17,949
Estimated fair value for held to maturity	1,850,210	1,842,281
Equity and other securities, Amortized Cost	455,286	449,008
Equity securities, Gross Unrealized Gains	13,012	25,094
Equity securities, Gross Unrealized Losses	203
Equity and other securities, Estimated Fair Value	468,095	474,102
Other securities, Amortized cost	386,303	381,117
Other securities, Estimated fair value	386,303	381,117
Total debt securities Amortized cost	5,973,021	6,370,016
Total debt securities Gross unrealized gains	267,554	321,890
Total debt securities Gross unrealized losses	25,928	27,019
Total debt securities Estimated fair value	6,214,647	6,664,887
Readily marketable securities Amortized cost	68,983	67,891
Readily marketable securities Gross unrealized gains	13,012	25,094
Readily marketable securities Gross unrealized losses	203
Readily marketable securities Estimated fair value	81,792	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	6,851	9,154
Investment securities available for sale, gross unrealized gains	163	198
Investment securities available for sale, gross unrealized losses	1	14
Investment securities available for sale, estimated fair value	7,013	9,338
Amortized cost for held to maturity	3,000	2,999
Estimated fair value for held to maturity	3,000	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	851	1,531
Gross unrealized gains for held to maturity	4	9
Estimated fair value for held to maturity	855	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	4,051,580	4,475,406
Investment securities available for sale, gross unrealized gains	174,154	208,787
Investment securities available for sale, gross unrealized losses	645	755
Investment securities available for sale, estimated fair value	4,225,089	4,683,438
Amortized cost for held to maturity	1,697,992	1,664,443
Gross unrealized gains for held to maturity	79,650	100,176
Gross unrealized losses for held to maturity	1,623	11
Estimated fair value for held to maturity	1,776,019	1,764,608
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	73,496	77,155
Gross unrealized gains for held to maturity	11,288	11,056
Gross unrealized losses for held to maturity	17,244	17,938
Estimated fair value for held to maturity	67,540	70,273
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	136,439	136,451
Investment securities available for sale, gross unrealized gains	2,295	1,664
Investment securities available for sale, gross unrealized losses	6,415	8,301
Investment securities available for sale, estimated fair value	132,319	129,814
Amortized cost for held to maturity	2,796	2,861
Estimated fair value for held to maturity	$ 2,796	$ 2,861

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