Fair Value Measurements (Details) (USD $)	6 Months Ended	12 Months Ended
Jun. 30, 2012	Dec. 31, 2011
Assets:
Available for sale securities	$ 5,208,603,000	$ 5,213,915,000
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	16,775,000	1,100,000
Write downs in fair value in connection with other real estate owned	668,000	9,509,000
Significant Unobservable Inputs (Level 3)
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3
Fair Value Measurements, Discounted Cash Flow Model Assumptions Default Rates, Thereafter		7.00%
Estimated future principal prepayment rate assumption, discount rate (as a percent)		13.00%
Loss severity rate assumptions, first year (as a percent)		60.00%
Decrease in loss severity rates, following three years (as a percent)		10.00%
Loss severity rate, thereafter (as a percent)		20.00%
Significant Unobservable Inputs (Level 3) \| Bond meeting the original contract terms
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3
Fair Value Measurements, Discounted Cash Flow Model Assumptions Default Rates, Thereafter	7.00%
Estimated future principal prepayment rate assumption, discount rate (as a percent)	13.00%
Default rate assumptions (as a percent)	1.00%
Loss severity rate assumptions, first year (as a percent)	25.00%
Significant Unobservable Inputs (Level 3) \| Bond not meeting the original contract terms
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3
Fair Value Measurements, Discounted Cash Flow Model Assumptions Default Rates, Thereafter	2.00%
Estimated future principal prepayment rate assumption, discount rate (as a percent)	13.00%
Default rate assumptions (as a percent)	9.00%
Loss severity rate assumptions, first year (as a percent)	60.00%
Decrease in loss severity rates, following five years (as a percent)	5.00%
Loss severity rate, thereafter (as a percent)	25.00%
Measured on a recurring basis: \| Assets/Liabilities Measured at Fair Value
Assets:
Available for sale securities	5,208,603,000	5,213,915,000
Measured on a recurring basis: \| Assets/Liabilities Measured at Fair Value \| Residential mortgage-backed securities
Assets:
Available for sale securities	4,953,594,000	4,969,263,000
Measured on a recurring basis: \| Assets/Liabilities Measured at Fair Value \| States and political subdivisions
Assets:
Available for sale securities	233,899,000	224,761,000
Measured on a recurring basis: \| Assets/Liabilities Measured at Fair Value \| Other
Assets:
Available for sale securities	21,110,000	19,891,000
Measured on a recurring basis: \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available for sale securities	21,110,000	19,891,000
Measured on a recurring basis: \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Other
Assets:
Available for sale securities	21,110,000	19,891,000
Measured on a recurring basis: \| Significant Other Observable Inputs (Level 2)
Assets:
Available for sale securities	5,151,617,000	5,154,419,000
Measured on a recurring basis: \| Significant Other Observable Inputs (Level 2) \| Residential mortgage-backed securities
Assets:
Available for sale securities	4,917,718,000	4,929,658,000
Measured on a recurring basis: \| Significant Other Observable Inputs (Level 2) \| States and political subdivisions
Assets:
Available for sale securities	233,899,000	224,761,000
Measured on a recurring basis: \| Significant Unobservable Inputs (Level 3)
Assets:
Available for sale securities	35,876,000	39,605,000
Measured on a recurring basis: \| Significant Unobservable Inputs (Level 3) \| Residential mortgage-backed securities
Assets:
Available for sale securities	35,876,000	39,605,000
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	(527,000)	15,457,000
Change in provision, other real estate owned	668,000	9,509,000
Measured on a non-recurring basis: \| Assets/Liabilities Measured at Fair Value
Assets:
Impaired Loans	12,709,000	81,723,000
Other real estate owned	13,407,000	34,631,000
Measured on a non-recurring basis: \| Significant Unobservable Inputs (Level 3)
Assets:
Impaired Loans	12,709,000	81,723,000
Other real estate owned	$ 10,924,000	$ 34,631,000
Maximum \| Significant Unobservable Inputs (Level 3)
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3
Fair Value Measurements, Discounted Cash Flow Model Assumptions Default Rates, Thereafter		5.00%
Default rate assumptions for the first year (as a percent)		11.00%
Minimum \| Significant Unobservable Inputs (Level 3)
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3
Fair Value Measurements, Discounted Cash Flow Model Assumptions Default Rates, Thereafter		4.00%
Default rate assumptions for the first year (as a percent)		10.00%

Reflects the amount of charge-off related to a loan prior to foreclosure.

No definition available.

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- Definition

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

+ References

No definition available.

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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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- Definition

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

+ References

No definition available.

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- Definition

The decline in the loss severity rate assumptions 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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No definition available.

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- Definition

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

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No definition available.

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Defaults 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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No definition available.

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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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No definition available.

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- Definition

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.

+ References

No definition available.

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- Definition

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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No definition available.

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- Definition

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

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No definition available.

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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.

+ References

No definition available.

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- Definition

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

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-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.

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 9

-Section 03

-Paragraph 7

-Subparagraph d

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)

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- Definition

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

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

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

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