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Fair Value Measurements (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Assets:
Available-for-sale	$ 4,988,913,000	$ 5,086,457,000
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3
Charges to allowance for probable loan losses in connection with other real estate owned	1,560,000	23,999,000
Write downs in fair value in connection with other real estate owned	7,998,000	(719,000)
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | U.S. Treasury securities
Assets:
Available-for-sale		1,327,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities
Assets:
Available-for-sale	4,749,581,000	4,924,468,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Obligations of states and political subdivisions
Assets:
Available-for-sale	224,515,000	145,997,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Equity Securities
Assets:
Available-for-sale	14,817,000	14,665,000
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Securities
Assets:
Available-for-sale	14,817,000	14,665,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Available-for-sale		1,327,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities
Assets:
Available-for-sale	4,709,104,000	4,878,440,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions
Assets:
Available-for-sale	224,515,000	145,997,000
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities
Assets:
Available-for-sale	40,477,000	46,028,000
Measured on a non-recurring basis:
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3
Change in provision, impaired loans	7,238,000	(114,000)
Change in provision, other real estate owned	7,998,000	719,000
Measured on a non-recurring basis: | Assets/Liabilities Measured at Fair Value
Assets:
Impaired Loans	60,588,000	24,034,000
Non-financial assets:
Other real estate owned	33,122,000	52,319,000
Measured on a non-recurring basis: | Significant Unobservable Inputs (Level 3)
Assets:
Impaired Loans	60,588,000	24,034,000
Non-financial assets:
Other real estate owned	$ 33,122,000	$ 52,319,000
Significant Unobservable Inputs (Level 3)
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3
Estimated future principal prepayment rate assumption, low end of range (as a percent)	4.00%
Estimated future principal prepayment rate assumption, high end of range (as a percent)	5.00%
Estimated future principal prepayment rate assumption, discount rate (as a percent)	13.00%
Default rate assumptions for the first year, low end of range (as a percent)	10.00%
Default rate assumptions for the first year, high end of range (as a percent)	11.00%
Default rate assumptions, thereafter (as a percent)	7.00%
Loss severity rate assumptions, first year (as a percent)	60.00%
Decrease in loss severity rates, following three years (as a percent)	10.00%
Decrease in loss severity rates, thereafter (as a percent)	20.00%

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The estimated fair value of recorded investment related to change in provision for impaired financing receivables measured at fair value.

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Carrying amount as of the balance sheet date of change in Provision of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings.

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The decline in the loss severity rate assumption for the following three years used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy.

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The high end of range of default rates assumption for the first year used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy.

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The low end of range of default rates assumption for the first year used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy.

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Default rates assumption for the remaining years used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy.

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The discount rate assumption of estimated future principal prepayment rate assumption used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy.

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The high end of range of estimated future principal prepayment rate assumption used in the discounted cash flow model for the investments classified within level 3 of the fair value hierarchy.

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The low end of range of estimated future principal prepayment rate assumption used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy.

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Loss severity rates assumption for the first year used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy.

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Loss severity rates assumption for the remaining years used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy.

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The estimated fair value of the recorded investment related to impaired financing receivables measured at fair value.

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Carrying amount as of the balance sheet date of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings.

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Reflects the amount charged against earnings during the period as bad debt expense, net of recoveries of previously expensed credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 114

-Paragraph 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 9

-Section 03

-Paragraph 7

-Subparagraph d



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

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