Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 6,258,276	$ 8,869,423
Investment securities available for sale, gross unrealized gains	90,723	16,528
Investment securities available for sale, gross unrealized losses	30,223	203,442
Investment securities available for sale, estimated fair value	6,318,776	8,682,509
Amortized cost for held to maturity	2,656,917	3,316,640
Gross unrealized gains for held to maturity	62,462	16,532
Gross unrealized losses for held to maturity	20,173	77,689
Estimated fair value for held to maturity	2,699,206	3,255,483
Equity and other securities, Amortized Cost	487,041	677,187
Equity securities, Gross Unrealized Gains	34,786	17,295
Equity securities, Gross Unrealized Losses	269	818
Equity and other securities, Estimated Fair Value	521,558	693,664
Other securities, Amortized cost	381,517	599,747
Other securities, Estimated fair value	381,517	599,747
Total debt securities Amortized cost	8,915,193	12,186,063
Total debt securities Gross unrealized gains	153,185	33,060
Total debt securities Gross unrealized losses	50,396	281,131
Total debt securities Estimated fair value	9,017,982	11,937,992
Readily marketable securities Amortized cost	105,524	77,440
Readily marketable securities Gross unrealized gains	34,786	17,295
Readily marketable securities Gross unrealized losses	269	818
Readily marketable securities Estimated fair value	140,041	93,917
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	9,742	1,346,782
Investment securities available for sale, gross unrealized gains	41
Investment securities available for sale, gross unrealized losses	16	9,851
Investment securities available for sale, estimated fair value	9,767	1,336,931
Amortized cost for held to maturity	249,862	446,542
Gross unrealized gains for held to maturity	286
Gross unrealized losses for held to maturity		239
Estimated fair value for held to maturity	250,148	446,303
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	1,660
Investment securities available for sale, gross unrealized gains	2	4
Investment securities available for sale, gross unrealized losses	3	5
Investment securities available for sale, estimated fair value	775	1,659
Amortized cost for held to maturity	4,140	7,494
Gross unrealized gains for held to maturity	16	22
Gross unrealized losses for held to maturity		12
Estimated fair value for held to maturity	4,156	7,504
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	6,113,913	7,383,340
Investment securities available for sale, gross unrealized gains	88,634	15,754
Investment securities available for sale, gross unrealized losses	21,607	182,103
Investment securities available for sale, estimated fair value	6,180,940	7,216,991
Amortized cost for held to maturity	2,306,180	2,745,776
Gross unrealized gains for held to maturity	50,381	4,165
Gross unrealized losses for held to maturity	1,992	55,111
Estimated fair value for held to maturity	2,354,569	2,694,830
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	16	24
Investment securities available for sale, gross unrealized losses		2
Investment securities available for sale, estimated fair value	16	22
Amortized cost for held to maturity	93,496	113,160
Gross unrealized gains for held to maturity	11,779	12,345
Gross unrealized losses for held to maturity	18,181	22,327
Estimated fair value for held to maturity	87,094	103,178
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	133,829	137,617
Investment securities available for sale, gross unrealized gains	2,046	770
Investment securities available for sale, gross unrealized losses	8,597	11,481
Investment securities available for sale, estimated fair value	127,278	126,906
Amortized cost for held to maturity	3,239	3,668
Estimated fair value for held to maturity	$ 3,239	$ 3,668

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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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