Allowance for Credit Losses - Impaired Loans (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	$ 2,875,000	$ 5,704,000
Upaid Principal Balance, With no related allowance recorded	3,722,000	6,110,000
Recorded Investment, With an allowance recorded	14,230,000	17,940,000
Unpaid Principal Balance, With an allowance recorded	15,336,000	18,822,000
Related Allowance	510,000	4,368,000
Total Recorded Investment	17,105,000	23,644,000
Total Unpaid Principal Balance	19,058,000	24,932,000
Average Recorded Investment, With no related allowance recorded	9,486,000	5,282,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	9,486,000	10,832,000
Interest Income Recognized, With an allowance recorded	601,000	411,000
Average Recorded Investment, Total	18,972,000	16,114,000
Interest Income Recognized, Total	601,000	411,000
Forgone interest on nonaccrual loans	693,000	954,000	1,228,000
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	2,140,000
Upaid Principal Balance, With no related allowance recorded	0	2,160,000
Recorded Investment, With an allowance recorded	2,405,000	1,717,000
Unpaid Principal Balance, With an allowance recorded	2,405,000	1,718,000
Related Allowance	40,000	231,000
Average Recorded Investment, With no related allowance recorded	952,000	544,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	1,581,000	505,000
Interest Income Recognized, With an allowance recorded	226,000	181,000
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	2,140,000
Upaid Principal Balance, With no related allowance recorded	0	2,160,000
Recorded Investment, With an allowance recorded	2,405,000	1,717,000
Unpaid Principal Balance, With an allowance recorded	2,405,000	1,718,000
Related Allowance	40,000	231,000
Average Recorded Investment, With no related allowance recorded	952,000	544,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	1,581,000	505,000
Interest Income Recognized, With an allowance recorded	226,000	181,000
Agricultural land and production
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	0
Upaid Principal Balance, With no related allowance recorded	0	0
Recorded Investment, With an allowance recorded	0	0
Unpaid Principal Balance, With an allowance recorded	0	0
Related Allowance	0	0
Average Recorded Investment, With no related allowance recorded	0	0
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	0	0
Interest Income Recognized, With an allowance recorded	0	0
Real estate
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	1,352,000	3,564,000
Upaid Principal Balance, With no related allowance recorded	1,888,000	3,950,000
Recorded Investment, With an allowance recorded	11,516,000	13,795,000
Unpaid Principal Balance, With an allowance recorded	12,608,000	14,449,000
Related Allowance	465,000	3,764,000
Average Recorded Investment, With no related allowance recorded	6,287,000	4,381,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	7,268,000	8,586,000
Interest Income Recognized, With an allowance recorded	375,000	230,000
Owner occupied
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	231,000
Upaid Principal Balance, With no related allowance recorded	0	243,000
Recorded Investment, With an allowance recorded	1,575,000	1,141,000
Unpaid Principal Balance, With an allowance recorded	1,733,000	1,216,000
Related Allowance	165,000	268,000
Average Recorded Investment, With no related allowance recorded	1,053,000	1,100,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	633,000	1,193,000
Interest Income Recognized, With an allowance recorded	0	0
Real estate construction and other land loans
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	1,352,000	1,532,000
Upaid Principal Balance, With no related allowance recorded	1,888,000	1,906,000
Recorded Investment, With an allowance recorded	9,941,000	10,911,000
Unpaid Principal Balance, With an allowance recorded	10,875,000	11,490,000
Related Allowance	300,000	2,130,000
Average Recorded Investment, With no related allowance recorded	4,933,000	1,690,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	6,490,000	6,544,000
Interest Income Recognized, With an allowance recorded	375,000	230,000
Commercial real estate
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	1,801,000
Upaid Principal Balance, With no related allowance recorded	0	1,801,000
Recorded Investment, With an allowance recorded	0	1,743,000
Unpaid Principal Balance, With an allowance recorded	0	1,743,000
Related Allowance	0	1,366,000
Average Recorded Investment, With no related allowance recorded	301,000	1,591,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	145,000	849,000
Interest Income Recognized, With an allowance recorded	0	0
Agricultural real estate
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	0
Upaid Principal Balance, With no related allowance recorded	0	0
Recorded Investment, With an allowance recorded	0	0
Unpaid Principal Balance, With an allowance recorded	0	0
Related Allowance	0	0
Average Recorded Investment, With no related allowance recorded	0	0
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	0	0
Interest Income Recognized, With an allowance recorded	0	0
Other real estate
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0
Upaid Principal Balance, With no related allowance recorded	0
Recorded Investment, With an allowance recorded	0	0
Unpaid Principal Balance, With an allowance recorded	0	0
Related Allowance	0	0
Average Recorded Investment, With no related allowance recorded	0	0
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	0	0
Interest Income Recognized, With an allowance recorded	0	0
Consumer
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	1,523,000	0
Upaid Principal Balance, With no related allowance recorded	1,834,000	0
Recorded Investment, With an allowance recorded	309,000	2,428,000
Unpaid Principal Balance, With an allowance recorded	323,000	2,655,000
Related Allowance	5,000	373,000
Average Recorded Investment, With no related allowance recorded	1,567,000	357,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	637,000	1,741,000
Interest Income Recognized, With an allowance recorded	0	0
Equity loans and lines of credit
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	1,523,000	0
Upaid Principal Balance, With no related allowance recorded	1,834,000	0
Recorded Investment, With an allowance recorded	309,000	2,354,000
Unpaid Principal Balance, With an allowance recorded	323,000	2,581,000
Related Allowance	5,000	350,000
Average Recorded Investment, With no related allowance recorded	1,561,000	357,000
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	600,000	1,640,000
Interest Income Recognized, With an allowance recorded	0	0
Consumer and installment
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	0
Upaid Principal Balance, With no related allowance recorded	0	0
Recorded Investment, With an allowance recorded	0	74,000
Unpaid Principal Balance, With an allowance recorded	0	74,000
Related Allowance	0	23,000
Average Recorded Investment, With no related allowance recorded	6,000	0
Interest Income Recognized, With no related allowance recorded	0	0
Average Recorded Investment, With an allowance recorded	37,000	101,000
Interest Income Recognized, With an allowance recorded	$ 0	$ 0

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5258-111524

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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

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Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953550-111524

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Amount of allowance for credit losses related to recorded investment.

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Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

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Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

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Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

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Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

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Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

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Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

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Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher OTS

-Name Federal Regulation (FR)

-Number Title 12

-Section 563c.102

-Paragraph 8

-Subparagraph j(ii)

-Chapter V

-Subsection I

-LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 15

-Paragraph 40

-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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