Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 9,449,975	$ 10,938,796
Investment securities available for sale, gross unrealized gains	11,538	85,154
Investment securities available for sale, gross unrealized losses	306,998	127,666
Investment securities available for sale, estimated fair value	9,154,515	10,896,284
Amortized cost for held to maturity	3,418,719	3,353,213
Gross unrealized gains for held to maturity	12,969	29,919
Gross unrealized losses for held to maturity	118,418	41,370
Estimated fair value for held to maturity	3,313,270	3,341,762
Equity and other securities, Amortized Cost	488,389	415,028
Equity securities, Gross Unrealized Gains	13,253
Equity securities, Gross Unrealized Losses	995
Equity and other securities, Estimated Fair Value	500,647	415,028
Other securities, Amortized cost	410,518	415,028
Other securities, Estimated fair value	410,518	415,028
Total debt securities Amortized cost	12,868,694
Total debt securities Gross unrealized gains	24,507
Total debt securities Gross unrealized losses	425,416
Total debt securities Estimated fair value	12,467,785
Readily marketable securities Amortized cost	77,871
Readily marketable securities Gross unrealized gains	13,253
Readily marketable securities Gross unrealized losses	995
Readily marketable securities Estimated fair value	90,129
Total Amortized cost		14,707,037
Total for Gross unrealized gains		115,073
Total for Gross unrealized losses		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	1,623,860	1,965,665
Investment securities available for sale, gross unrealized gains	8
Investment securities available for sale, gross unrealized losses	15,196	18,178
Investment securities available for sale, estimated fair value	1,608,672	1,947,487
Amortized cost for held to maturity	445,199
Gross unrealized losses for held to maturity	412
Estimated fair value for held to maturity	444,787
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	1,792	2,555
Investment securities available for sale, gross unrealized gains	9	36
Investment securities available for sale, gross unrealized losses	5	2
Investment securities available for sale, estimated fair value	1,796	2,589
Amortized cost for held to maturity	8,752	24,562
Gross unrealized gains for held to maturity	32	109
Gross unrealized losses for held to maturity	18	49
Estimated fair value for held to maturity	8,766	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	7,686,746	8,755,482
Investment securities available for sale, gross unrealized gains	9,610	59,497
Investment securities available for sale, gross unrealized losses	283,660	98,587
Investment securities available for sale, estimated fair value	7,412,696	8,716,392
Amortized cost for held to maturity	2,842,807	3,187,953
Gross unrealized gains for held to maturity	1,912	27,236
Gross unrealized losses for held to maturity	94,241	13,746
Estimated fair value for held to maturity	2,750,478	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	25	28
Investment securities available for sale, gross unrealized losses	2
Investment securities available for sale, estimated fair value	23	28
Amortized cost for held to maturity	117,996	135,688
Gross unrealized gains for held to maturity	11,025	2,574
Gross unrealized losses for held to maturity	23,747	27,575
Estimated fair value for held to maturity	105,274	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	137,552	136,905
Investment securities available for sale, gross unrealized gains	1,911	2,402
Investment securities available for sale, gross unrealized losses	8,135	10,475
Investment securities available for sale, estimated fair value	131,328	128,832
Amortized cost for held to maturity	3,965	5,010
Estimated fair value for held to maturity	$ 3,965	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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Investment in debt securities amortized cost.

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Investment in debt securities gross unrealized gains.

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Total amount of accumulated unrealized gain on investments in debt and equity securities before taxes.

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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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