Investment securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Other securities, Amortized Cost	$ 318,801	$ 366,886
Other securities, Gross Unrealized Gains
Other securities, Gross Unrealized Loss
Other securities, Estimated fair value	318,801	366,886
Investment securities, available for sale, amortized cost	5,091,116	6,312,423
Gross unrealized gains	279,693	210,001
Gross unrealized losses	186,931	293,864
Estimated fair value	5,183,878	6,228,560
Amortized cost for held to maturity	1,121,325	1,077,708
Gross unrealized gains for held to maturity	46,441	34,022
Gross unrealized losses for held to maturity	92,995	99,168
Estimated fair value for held to maturity	1,074,771	1,012,562
Total Amortized cost	6,531,242	7,757,017
Total for Gross unrealized gains	326,134	244,023
Total for Gross unrealized losses	279,926	393,032
Total for Estimated fair value	6,577,450	7,608,008
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	33,402	39,518
Gross unrealized gains	588	771
Gross unrealized losses	10	20
Estimated fair value	33,980	40,269
Amortized cost for held to maturity	192,621	188,680
Gross unrealized gains for held to maturity	8,848	9,141
Gross unrealized losses for held to maturity	29	28
Estimated fair value for held to maturity	201,440	197,793
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	3,502,237	4,344,116
Gross unrealized gains	246,190	177,392
Gross unrealized losses	175	275
Estimated fair value	3,748,252	4,521,233
Amortized cost for held to maturity	669,169	608,533
Gross unrealized gains for held to maturity	37,259	24,881
Estimated fair value for held to maturity	706,428	633,414
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	248,441	268,642
Gross unrealized gains for held to maturity	334
Gross unrealized losses for held to maturity	92,966	99,140
Estimated fair value for held to maturity	155,809	169,502
Mortgage-backed securities [Member] \| Privately issued residential [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	1,198,896	1,369,371
Gross unrealized gains	4,929	6,373
Gross unrealized losses	148,757	239,488
Estimated fair value	1,055,068	1,136,256
Mortgage-backed securities [Member] \| Privately issued commercial [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	10,837	17,679
Gross unrealized losses	269	2,650
Estimated fair value	10,568	15,029
Mortgage-backed securities [Member] \| Collateralized debt obligations [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	43,294	43,834
Gross unrealized gains	15,147	11,154
Gross unrealized losses	1,240	2,488
Estimated fair value	57,201	52,500
Other debt securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	136,418	216,700
Gross unrealized gains	2,146	4,588
Gross unrealized losses	29,159	44,443
Estimated fair value	109,405	176,845
Amortized cost for held to maturity	11,094	11,853
Estimated fair value for held to maturity	11,094	11,853
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	55,288	69,468
Gross unrealized gains	1,057	1,255
Estimated fair value	56,345	70,723
Equity securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	110,744	211,737
Gross unrealized gains	9,636	8,468
Gross unrealized losses	7,321	4,500
Estimated fair value	$ 113,059	$ 215,705

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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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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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