Securities Sold Under Repurchase Agreements (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Securities Sold Under Repurchase Agreements
Average outstanding amount	$ 1,276,831,000	$ 1,415,769,000
Maximum amount outstanding at any month end	1,412,744,000	1,437,480,000
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	1,418,665,000	1,672,925,000
Collateralized securities, Fair Value of Securities Sold	1,459,837,000	1,736,519,000
Repurchase Borrowing, Balance of Liability	1,129,679,000	1,348,629,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	2.85%	3.02%
Overnight agreements
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	340,711,000	377,620,000
Collateralized securities, Fair Value of Securities Sold	350,933,000	388,304,000
Repurchase Borrowing, Balance of Liability	263,992,000	245,026,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.28%	0.32%
1 to 29 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	10,378,000	13,772,000
Collateralized securities, Fair Value of Securities Sold	10,910,000	14,227,000
Repurchase Borrowing, Balance of Liability	6,992,000	10,263,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	1.10%	1.13%
30 to 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	33,619,000	45,512,000
Collateralized securities, Fair Value of Securities Sold	35,097,000	48,177,000
Repurchase Borrowing, Balance of Liability	22,078,000	30,095,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	1.05%	1.12%
Over 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	1,033,957,000	1,236,021,000
Collateralized securities, Fair Value of Securities Sold	1,062,897,000	1,285,811,000
Repurchase Borrowing, Balance of Liability	$ 836,617,000	$ 1,063,245,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	3.73%	3.72%

Represents the average amount of securities sold under repurchase agreements during the period.

No definition available.

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- Definition

Represents the maximum balance outstanding at any month end during the period as of the balance sheet date.

+ References

No definition available.

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- Definition

Carrying amount of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(i))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph m

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- Definition

Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

+ References

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

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-Number 210

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Market value of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

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- Definition

The carrying value as of the balance sheet date of securities that an institution sells and agrees to repurchase (the identical or substantially the same securities) as a seller-borrower at a specified date for a specified price, also known as a repurchase agreement, or repo. Most repos involve obligations of the federal government or its agencies, but other financial instruments, such as commercial paper, banker's acceptances, and negotiable certificates of deposit, are sometimes used in repos.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 140

-Paragraph 100

-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 FASB

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-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

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-IssueDate 2006-05-01

-Chapter 4

-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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