Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 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	$ 9,912,160	$ 10,938,796
Investment securities available for sale, gross unrealized gains	17,762	85,154
Investment securities available for sale, gross unrealized losses	271,076	127,666
Investment securities available for sale, estimated fair value	9,658,846	10,896,284
Amortized cost for held to maturity	3,101,095	3,353,213
Gross unrealized gains for held to maturity	16,047	29,919
Gross unrealized losses for held to maturity	94,813	41,370
Estimated fair value for held to maturity	3,022,329	3,341,762
Equity and other securities, Amortized Cost	507,389	415,028
Equity securities, Gross Unrealized Gains	16,527
Equity securities, Gross Unrealized Losses	855
Equity and other securities, Estimated Fair Value	523,061	415,028
Other securities, Amortized cost	436,579	415,028
Other securities, Estimated fair value	436,579	415,028
Total debt securities Amortized cost	13,013,255
Total debt securities Gross unrealized gains	33,809
Total debt securities Gross unrealized losses	365,889
Total debt securities Estimated fair value	12,681,175
Readily marketable securities Amortized cost	70,810
Readily marketable securities Gross unrealized gains	16,527
Readily marketable securities Gross unrealized losses	855
Readily marketable securities Estimated fair value	86,482
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	1,726,242	1,965,665
Investment securities available for sale, gross unrealized gains	4
Investment securities available for sale, gross unrealized losses	18,425	18,178
Investment securities available for sale, estimated fair value	1,707,821	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	1,914	2,555
Investment securities available for sale, gross unrealized gains	12	36
Investment securities available for sale, gross unrealized losses	1	2
Investment securities available for sale, estimated fair value	1,925	2,589
Amortized cost for held to maturity	13,547	24,562
Gross unrealized gains for held to maturity	57	109
Gross unrealized losses for held to maturity	20	49
Estimated fair value for held to maturity	13,584	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	8,046,603	8,755,482
Investment securities available for sale, gross unrealized gains	15,521	59,497
Investment securities available for sale, gross unrealized losses	244,825	98,587
Investment securities available for sale, estimated fair value	7,817,299	8,716,392
Amortized cost for held to maturity	2,960,045	3,187,953
Gross unrealized gains for held to maturity	4,539	27,236
Gross unrealized losses for held to maturity	71,183	13,746
Estimated fair value for held to maturity	2,893,401	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	26	28
Investment securities available for sale, gross unrealized losses	2
Investment securities available for sale, estimated fair value	24	28
Amortized cost for held to maturity	122,906	135,688
Gross unrealized gains for held to maturity	11,451	2,574
Gross unrealized losses for held to maturity	23,610	27,575
Estimated fair value for held to maturity	110,747	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	137,375	136,905
Investment securities available for sale, gross unrealized gains	2,225	2,402
Investment securities available for sale, gross unrealized losses	7,823	10,475
Investment securities available for sale, estimated fair value	131,777	128,832
Amortized cost for held to maturity	4,597	5,010
Estimated fair value for held to maturity	$ 4,597	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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Equity securities gross unrealized losses.

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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 gains.

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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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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 gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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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 of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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