Loans and Allowance - Composition of Impaired Loans by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 109,594	$ 98,922	$ 120,104
Recorded Investment	67,333	59,835	76,317
Average Recorded Investment	76,517	69,330
Interest Income Recognized	1,498	1,241
Unpaid Principal Balance	12,918	24,234	42,488
Recorded Investment	11,846	19,940	39,887
Related Allowance	4,243	7,596	13,914
Average Recorded Investment	12,097	21,462
Interest Income Recognized	203	321
Impaired Financing Receivable, Unpaid Principal Balance	122,512	123,156	162,592
Impaired Financing Receivable, Recorded Investment	79,179	79,775	116,204
Average Recorded Investment	88,614	90,792
Interest Income Recognized	1,701	1,562
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	28,532	23,364	30,006
Recorded Investment	11,730	10,116	16,572
Average Recorded Investment	15,089	13,399
Interest Income Recognized	124	615
Unpaid Principal Balance	4,415	8,691	11,477
Recorded Investment	4,155	8,104	11,374
Related Allowance	1,628	4,142	5,250
Average Recorded Investment	4,225	8,196
Interest Income Recognized	33	174
Agriculture production financing and other loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance			966
Recorded Investment			530
Construction
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	9,787	14,301	12,598
Recorded Investment	5,164	7,701	9,150
Average Recorded Investment	6,471	8,836
Interest Income Recognized	66
Unpaid Principal Balance	1,202	961	9,353
Recorded Investment	1,058	961	7,824
Related Allowance	105	321	2,049
Average Recorded Investment	1,175	961
Commercial and farm land
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	58,173	49,242	64,064
Recorded Investment	43,204	34,571	43,653
Average Recorded Investment	46,788	39,032
Interest Income Recognized	1,211	591
Unpaid Principal Balance	5,579	12,115	17,984
Recorded Investment	5,182	8,748	17,076
Related Allowance	2,460	2,183	5,496
Average Recorded Investment	5,239	10,028
Interest Income Recognized	95	140
Residential
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	8,820	7,491	7,909
Recorded Investment	6,215	6,185	5,153
Average Recorded Investment	7,129	6,539
Interest Income Recognized	83	20
Unpaid Principal Balance	1,722	1,888	2,740
Recorded Investment	1,451	1,575	2,691
Related Allowance	50	391	465
Average Recorded Investment	1,458	1,687
Interest Income Recognized	75	7
Home Equity
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	4,199	4,425	4,460
Recorded Investment	1,006	1,241	1,245
Average Recorded Investment	1,022	1,500
Interest Income Recognized	13	15
Unpaid Principal Balance			458
Recorded Investment			446
Related Allowance			178
Other Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	83	99	101
Recorded Investment	14	21	14
Average Recorded Investment	18	24
Interest Income Recognized	1
Unpaid Principal Balance		579	476
Recorded Investment		552	476
Related Allowance		559	476
Average Recorded Investment		$ 590

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

+ Details

Name:	us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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=7512638&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant