Investment securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Other securities, Amortized cost	$ 305,749	$ 302,648
Other securities, Gross unrealized gains
Other securities, Gross unrealized losses
Other securities, Estimated fair value	305,749	302,648
Investment securities, available for sale, amortized cost	2,985,526	4,643,070
Gross unrealized gains	123,093	253,619
Gross unrealized losses	23,533	157,252
Estimated fair value	3,085,086	4,739,437
Amortized cost for held to maturity	1,819,691	1,032,276
Gross unrealized gains for held to maturity	16,894	39,534
Gross unrealized losses for held to maturity	99,007	94,927
Estimated fair value for held to maturity	1,737,578	976,883
Total Amortized cost	5,110,966	5,977,994
Total for Gross unrealized gains	139,987	293,153
Total for Gross unrealized losses	122,540	252,179
Total for Estimated fair value	5,128,413	6,018,968
Collateralized Debt Obligations [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	42,911	43,228
Gross unrealized gains	18,140	19,663
Gross unrealized losses	1,135	1,022
Estimated fair value	59,916	61,869
Obligations of States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	18,874	20,375
Gross unrealized gains	364	534
Gross unrealized losses	7	8
Estimated fair value	19,231	20,901
Amortized cost for held to maturity	177,824	182,103
Gross unrealized gains for held to maturity	4,355	7,647
Gross unrealized losses for held to maturity	73	27
Estimated fair value for held to maturity	182,106	189,723
Mortgage-backed Securities [Member] \| Government Issued or Guaranteed [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	2,612,466	3,163,210
Gross unrealized gains	83,452	208,060
Gross unrealized losses	1,025	229
Estimated fair value	2,694,893	3,371,041
Amortized cost for held to maturity	1,401,569	597,340
Gross unrealized gains for held to maturity	12,517	31,727
Gross unrealized losses for held to maturity	30,841
Estimated fair value for held to maturity	1,383,245	629,067
Mortgage-backed Securities [Member] \| Privately issued [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Amortized cost for held to maturity	230,625	242,378
Gross unrealized gains for held to maturity	22	160
Gross unrealized losses for held to maturity	68,093	94,900
Estimated fair value for held to maturity	162,554	147,638
Mortgage-backed Securities [Member] \| Privately Issued Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	4,771	1,142,287
Gross unrealized gains	525	7,272
Gross unrealized losses	24	125,673
Estimated fair value	5,272	1,023,886
Other Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	136,841	136,603
Gross unrealized gains	1,760	2,247
Gross unrealized losses	20,650	26,900
Estimated fair value	117,951	111,950
Amortized cost for held to maturity	9,673	10,455
Gross unrealized gains for held to maturity
Gross unrealized losses for held to maturity
Estimated fair value for held to maturity	9,673	10,455
U.S. Treasury and Federal Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	37,486	38,422
Gross unrealized gains	622	922
Gross unrealized losses
Estimated fair value	38,108	39,344
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	132,177	98,945
Gross unrealized gains	18,230	14,921
Gross unrealized losses	692	3,420
Estimated fair value	$ 149,715	$ 110,446

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Investment Securities Fair Value.

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Line item includes holdings of Federal Home Loan Bank and Federal Reserve Bank stock (at amortized cost).

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Parenthetical disclosure of holdings of Federal Home Loan Bank and Federal Reserve Bank Stock (at fair value).

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

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For an unclassified balance sheet, this item represents investments in debt securities which are categorized as held-to-maturity. The held-to-maturity category is for those securities that the entity has the positive intent and ability to hold until maturity.

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As of the balance sheet date, the excess of fair value over carrying value (amortized cost) of a debt security categorized as held-to-maturity, exclusive of dividend or interest income recognized but not yet received and exclusive of any write-downs for other-than-temporary impairment. Such item represents the gross unrecognized holding gain.

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