Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 8,869,423	$ 10,938,796
Investment securities available for sale, gross unrealized gains	16,528	85,154
Investment securities available for sale, gross unrealized losses	203,442	127,666
Investment securities available for sale, estimated fair value	8,682,509	10,896,284
Amortized cost for held to maturity	3,316,640	3,353,213
Gross unrealized gains for held to maturity	16,532	29,919
Gross unrealized losses for held to maturity	77,689	41,370
Estimated fair value for held to maturity	3,255,483	3,341,762
Equity and other securities, Amortized Cost	677,187	415,028
Equity securities, Gross Unrealized Gains	17,295
Equity securities, Gross Unrealized Losses	818
Equity and other securities, Estimated Fair Value	693,664	415,028
Other securities, Amortized cost	599,747	415,028
Other securities, Estimated fair value	599,747	415,028
Total debt securities Amortized cost	12,186,063
Total debt securities Gross unrealized gains	33,060
Total debt securities Gross unrealized losses	281,131
Total debt securities Estimated fair value	11,937,992
Readily marketable securities Amortized cost	77,440
Readily marketable securities Gross unrealized gains	17,295
Readily marketable securities Gross unrealized losses	818
Readily marketable securities Estimated fair value	93,917
Total Amortized cost		14,707,037
Total for Gross unrealized gains		115,073
Total for Gross unrealized losses	281,000	169,036
Total for Estimated fair value		14,653,074
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	1,346,782	1,965,665
Investment securities available for sale, gross unrealized losses	9,851	18,178
Investment securities available for sale, estimated fair value	1,336,931	1,947,487
Amortized cost for held to maturity	446,542
Gross unrealized losses for held to maturity	239
Estimated fair value for held to maturity	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	1,660	2,555
Investment securities available for sale, gross unrealized gains	4	36
Investment securities available for sale, gross unrealized losses	5	2
Investment securities available for sale, estimated fair value	1,659	2,589
Amortized cost for held to maturity	7,494	24,562
Gross unrealized gains for held to maturity	22	109
Gross unrealized losses for held to maturity	12	49
Estimated fair value for held to maturity	7,504	24,622
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	7,383,340	8,755,482
Investment securities available for sale, gross unrealized gains	15,754	59,497
Investment securities available for sale, gross unrealized losses	182,103	98,587
Investment securities available for sale, estimated fair value	7,216,991	8,716,392
Amortized cost for held to maturity	2,745,776	3,187,953
Gross unrealized gains for held to maturity	4,165	27,236
Gross unrealized losses for held to maturity	55,111	13,746
Estimated fair value for held to maturity	2,694,830	3,201,443
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	24	28
Investment securities available for sale, gross unrealized losses	2
Investment securities available for sale, estimated fair value	22	28
Amortized cost for held to maturity	113,160	135,688
Gross unrealized gains for held to maturity	12,345	2,574
Gross unrealized losses for held to maturity	22,327	27,575
Estimated fair value for held to maturity	103,178	110,687
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	137,617	136,905
Investment securities available for sale, gross unrealized gains	770	2,402
Investment securities available for sale, gross unrealized losses	11,481	10,475
Investment securities available for sale, estimated fair value	126,906	128,832
Amortized cost for held to maturity	3,668	5,010
Estimated fair value for held to maturity	$ 3,668	5,010
Equity Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost		78,161
Investment securities available for sale, gross unrealized gains		23,219
Investment securities available for sale, gross unrealized losses		424
Investment securities available for sale, estimated fair value		$ 100,956

Equity and other securities at cost.

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Readily marketable securities at estimated fair value.

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Readily marketable securities at fair value amortized cost.

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Readily marketable securities at fair value gross unrealized losses.

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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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