Composition of Impaired Loans by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 104,654	$ 104,654	$ 98,922
Unpaid Principal Balance	125,548	125,548	123,156
Recorded Investment	60,470	60,470	59,835
Recorded Investment	79,965	79,965	79,775
Related Allowance
Average Recorded Investment	55,699	61,126	69,330
Interest Income Recognized	316	886	1,241
Unpaid Principal Balance	20,894	20,894	24,234
Recorded Investment	19,495	19,495	19,940
Related Allowance	6,251	6,251	7,596
Average Recorded Investment	19,565	19,704	21,462
Average Recorded Investment	75,264	80,830	90,792
Interest Income Recognized	62	183	321
Interest Income Recognized	378	1,069	1,562
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	27,363	27,363	23,364
Recorded Investment	10,743	10,743	10,116
Related Allowance
Average Recorded Investment	9,994	11,615	13,399
Interest Income Recognized	26	68	615
Unpaid Principal Balance	5,570	5,570	8,691
Recorded Investment	5,227	5,227	8,104
Related Allowance	1,775	1,775	4,142
Average Recorded Investment	5,244	5,307	8,196
Interest Income Recognized	11	32	174
Agriculture production financing and other loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	300	300
Recorded Investment	300	300
Related Allowance
Average Recorded Investment	300	300
Construction
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	10,743	10,743	14,301
Recorded Investment	6,378	6,378	7,701
Related Allowance
Average Recorded Investment	6,658	7,665	8,836
Interest Income Recognized	17	47
Unpaid Principal Balance	725	725	961
Recorded Investment	480	480	961
Related Allowance	51	51	321
Average Recorded Investment	510	541	961
Commercial and farm land
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	52,953	52,953	49,242
Recorded Investment	35,606	35,606	34,571
Related Allowance
Average Recorded Investment	31,081	33,459	39,032
Interest Income Recognized	250	706	591
Unpaid Principal Balance	13,021	13,021	12,115
Recorded Investment	12,309	12,309	8,748
Related Allowance	4,106	4,106	2,183
Average Recorded Investment	12,326	12,371	10,028
Interest Income Recognized	32	95	140
Residential
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	9,307	9,307	7,491
Recorded Investment	6,716	6,716	6,185
Related Allowance
Average Recorded Investment	6,936	7,342	6,539
Interest Income Recognized	20	54	20
Unpaid Principal Balance	1,578	1,578	1,888
Recorded Investment	1,479	1,479	1,575
Related Allowance	319	319	391
Average Recorded Investment	1,485	1,485	1,687
Interest Income Recognized	19	56	7
Home Equity
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	3,901	3,901	4,425
Recorded Investment	711	711	1,241
Related Allowance
Average Recorded Investment	713	727	1,500
Interest Income Recognized	3	10	15
Other Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	87	87	99
Recorded Investment	16	16	21
Related Allowance
Average Recorded Investment	17	18	24
Interest Income Recognized		1
Unpaid Principal Balance			579
Recorded Investment			552
Related Allowance			559
Average Recorded Investment			$ 590

Impaired Financing Receivable, With No Related Allowance, Related Allowance

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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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+ 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 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 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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