Investment securities (Details) (USD $) In Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Investment securities available for sale:
Investment securities, available for sale, amortized cost	$ 5,026,881	$ 5,494,377
Gross Unrealized Gains	111,689	230,852
Gross Unrealized Losses	248,081	311,737
Estimated fair value	4,890,489	5,413,492
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	1,219,686	1,324,339
Gross Unrealized Gains for Held-to-maturity	26,574	16,005
Gross Unrealized Losses for Held-to-maturity	102,764	115,091
Estimated fair value for Held-to-maturity	1,143,496	1,225,253
Amortized cost and estimated fair value of investment securities
Other securities, Amortized Cost	382,090	412,709
Other securities, Estimated fair value	382,090	412,709
Amortized Cost	6,628,657	7,231,425
Total for Gross Unrealized Gains	138,263	246,857
Total for Gross Unrealized losses	350,845	426,828
Total for Estimated fair value	6,416,075	7,051,454
U.S. Treasury and federal agencies [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	111,836	61,772
Gross Unrealized Gains	1,119	1,680
Gross Unrealized Losses	2	18
Estimated fair value	112,953	63,434
Obligations of states and political subdivisions [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	66,709	59,921
Gross Unrealized Gains	821	561
Gross Unrealized Losses	39	57
Estimated fair value	67,491	60,425
Obligations of states and political subdivisions [Member]
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	184,595	191,119
Gross Unrealized Gains for Held-to-maturity	4,239	1,944
Gross Unrealized Losses for Held-to-maturity	149	694
Estimated fair value for Held-to-maturity	188,685	192,369
Government issued or guaranteed [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	2,965,034	3,146,054
Gross Unrealized Gains	60,965	161,298
Gross Unrealized Losses	2,981	1,111
Estimated fair value	3,023,018	3,306,241
Government issued or guaranteed [Member]
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	730,785	808,108
Gross Unrealized Gains for Held-to-maturity	21,978	14,061
Gross Unrealized Losses for Held-to-maturity	0
Estimated fair value for Held-to-maturity	752,763	822,169
Privately issued [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	1,507,748	1,677,064
Gross Unrealized Gains	9,720	10,578
Gross Unrealized Losses	211,266	252,081
Estimated fair value	1,306,202	1,435,561
Privately issued [Member]
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	292,034	312,537
Gross Unrealized Gains for Held-to-maturity	357
Gross Unrealized Losses for Held-to-maturity	102,615	114,397
Estimated fair value for Held-to-maturity	189,776	198,140
Privately issued commercial [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	21,094	25,357
Gross Unrealized Gains	0
Gross Unrealized Losses	3,861	2,950
Estimated fair value	17,233	22,407
Collateralized Debt Obligations [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	44,268	95,080
Gross Unrealized Gains	18,800	24,754
Gross Unrealized Losses	1,467	9,078
Estimated fair value	61,601	110,756
Other Debt Securities [Member]
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	12,272	12,575
Gross Unrealized Gains for Held-to-maturity	0
Gross Unrealized Losses for Held-to-maturity	0
Estimated fair value for Held-to-maturity	12,272	12,575
Other Debt Securities [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	217,812	310,017
Gross Unrealized Gains	5,738	26,883
Gross Unrealized Losses	28,014	38,000
Estimated fair value	195,536	298,900
Equity Securities [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	92,380	119,112
Gross Unrealized Gains	14,526	5,098
Gross Unrealized Losses	451	8,442
Estimated fair value	$ 106,455	$ 115,768

Amortized cost and estimated fair value of investment securities.

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Investment Securities Amortized Cost.

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Investment Securities Fair Value.

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Investment Securities Gross Unrealized Gains.

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Investment Securities Gross Unrealized Losses.

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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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For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading. Such securities are reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain or loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 16

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 22

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
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-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13

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-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name FASB Staff Position (FSP)
-Number FAS115-1/124-1
-Paragraph 7
-Footnote 2

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-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 19

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This item represents the gross unrealized gains for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized gains are the excess of the fair value of the Available-for-sale Securities over their carrying value as of the reporting date. Such gross unrealized gains are included in other comprehensive income in the statement of shareholders' equity, unless the Available-for-sale Security is designated as a hedge. All or a portion of the unrealized holding gain of an available-for-sale security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 22

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13, 19

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This item represents the gross unrealized losses for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized losses are the excess of the carrying value of the Available-for-sale Securities over their fair value as of the reporting date. Such gross unrealized losses are included in other comprehensive income in the statement of shareholders' equity, unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

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For an unclassified balance sheet, this item represents investments in debt securities which are categorized as held-to-maturity; such investments are measured at amortized cost (carrying value). The held-to-maturity category is for those securities that the Entity has the positive intent and ability to hold until maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 43
-Chapter 3
-Section A
-Paragraph 4, 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 17

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
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This item represents the aggregate fair value of investments in debt securities as of the balance sheet date and which are categorized as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 19

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 137

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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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As of the balance sheet date, the excess of carrying value (amortized cost) over fair value 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 loss.

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