Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 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	$ 10,508,620	$ 10,938,796
Investment securities available for sale, gross unrealized gains	23,791	85,154
Investment securities available for sale, gross unrealized losses	234,555	127,666
Investment securities available for sale, estimated fair value	10,297,856	10,896,284
Amortized cost for held to maturity	3,228,756	3,353,213
Gross unrealized gains for held to maturity	14,160	29,919
Gross unrealized losses for held to maturity	82,701	41,370
Estimated fair value for held to maturity	3,160,215	3,341,762
Equity and other securities, Amortized Cost	526,593	415,028
Equity securities, Gross Unrealized Gains	14,075
Equity securities, Gross Unrealized Losses	716
Equity and other securities, Estimated Fair Value	539,952	415,028
Other securities, Amortized cost	478,235	415,028
Other securities, Estimated fair value	478,235	415,028
Total debt securities Amortized cost	13,737,376
Total debt securities Gross unrealized gains	37,951
Total debt securities Gross unrealized losses	317,256
Total debt securities Estimated fair value	13,458,071
Readily marketable securities Amortized cost	48,358
Readily marketable securities Gross unrealized gains	14,075
Readily marketable securities Gross unrealized losses	716
Readily marketable securities Estimated fair value	61,717
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	1,962,160	1,965,665
Investment securities available for sale, gross unrealized losses	20,682	18,178
Investment securities available for sale, estimated fair value	1,941,478	1,947,487
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	1,986	2,555
Investment securities available for sale, gross unrealized gains	16	36
Investment securities available for sale, gross unrealized losses	2	2
Investment securities available for sale, estimated fair value	2,000	2,589
Amortized cost for held to maturity	20,209	24,562
Gross unrealized gains for held to maturity	75	109
Gross unrealized losses for held to maturity	36	49
Estimated fair value for held to maturity	20,248	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	8,407,476	8,755,482
Investment securities available for sale, gross unrealized gains	21,318	59,497
Investment securities available for sale, gross unrealized losses	206,665	98,587
Investment securities available for sale, estimated fair value	8,222,129	8,716,392
Amortized cost for held to maturity	3,074,830	3,187,953
Gross unrealized gains for held to maturity	6,153	27,236
Gross unrealized losses for held to maturity	58,581	13,746
Estimated fair value for held to maturity	3,022,402	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	27	28
Investment securities available for sale, estimated fair value	27	28
Amortized cost for held to maturity	128,997	135,688
Gross unrealized gains for held to maturity	7,932	2,574
Gross unrealized losses for held to maturity	24,084	27,575
Estimated fair value for held to maturity	112,845	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	136,971	136,905
Investment securities available for sale, gross unrealized gains	2,457	2,402
Investment securities available for sale, gross unrealized losses	7,206	10,475
Investment securities available for sale, estimated fair value	132,222	128,832
Amortized cost for held to maturity	4,720	5,010
Estimated fair value for held to maturity	$ 4,720	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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Equity securities gross unrealized gains.

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Investment in debt securities amortized cost.

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Investment in debt securities estimated fair value.

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Investment in debt securities gross unrealized losses.

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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 gains.

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Readily marketable securities at fair value gross unrealized losses.

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Schedule of amortized cost and estimated fair value of investment securities.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of Federal Home Loan Bank (FHLB) and Federal Reserve Bank stock.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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