Loans and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	$ 26,941,000	$ 37,724,000
Recorded Investment With No Allowance	5,566,000	7,290,000
Recorded Investment With Allowance	14,628,000	20,636,000
Total Recorded Investment	20,194,000	27,926,000
Related Allowance	4,213,000	5,338,000
Average Recorded Investment	24,497,000	31,293,000	24,025,000
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	177,000	139,000
Recorded Investment With No Allowance		17,000
Recorded Investment With Allowance	165,000	114,000
Total Recorded Investment	165,000	131,000
Related Allowance	57,000	41,000
Average Recorded Investment	179,000	144,000
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	22,177,000	31,704,000
Recorded Investment With No Allowance	4,859,000	5,794,000
Recorded Investment With Allowance	11,359,000	16,754,000
Total Recorded Investment	16,218,000	22,548,000
Related Allowance	2,867,000	4,006,000
Average Recorded Investment	19,837,000	25,060,000
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	3,749,000	4,764,000
Recorded Investment With No Allowance	287,000	934,000
Recorded Investment With Allowance	2,906,000	3,347,000
Total Recorded Investment	3,193,000	4,281,000
Related Allowance	1,171,000	1,079,000
Average Recorded Investment	3,698,000	5,017,000
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	838,000	1,117,000
Recorded Investment With No Allowance	420,000	545,000
Recorded Investment With Allowance	198,000	421,000
Total Recorded Investment	618,000	966,000
Related Allowance	118,000	212,000
Average Recorded Investment	$ 783,000	$ 1,072,000

Impaired financing receivable, recorded investment with allowance.

No definition available.

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- Definition

Impaired financing receivable, recorded investment with no allowance.

+ References

No definition available.

+ Details

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- Definition

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=28368275&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=28368275&loc=d3e5258-111524

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- Definition

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=28368275&loc=SL6953550-111524

+ Details

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- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

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- Definition

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

+ References

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