Composition of Impaired Loans by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	$ 90,290		$ 109,594
Impaired loans with related allowance - Unpaid Principal Balance	15,376		12,918
Unpaid Principal Balance	105,666		122,512
Impaired loans with no related allowance - Recorded Investment	55,000		67,333
Impaired loans with related allowance - Recorded Investment	13,077		11,846
Recorded Investment	68,077		79,179
Impaired loans with related allowance - Related Allowance	3,622		4,243
Impaired loans with no related allowance - Average Recorded Investment	56,963	79,999
Impaired loans with related allowance - Average Recorded Investment	13,857	16,896
Average Recorded Investment	70,820	96,895
Impaired loans with no related allowance - Interest Income Recognized	470	371
Impaired loans with related allowance - Interest Income Recognized	3	47
Interest Income Recognized	473	418
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	20,987		28,532
Impaired loans with related allowance - Unpaid Principal Balance	5,751		4,415
Impaired loans with no related allowance - Recorded Investment	7,628		11,730
Impaired loans with related allowance - Recorded Investment	4,370		4,155
Impaired loans with related allowance - Related Allowance	838		1,628
Impaired loans with no related allowance - Average Recorded Investment	8,043	13,785
Impaired loans with related allowance - Average Recorded Investment	5,124	7,188
Impaired loans with no related allowance - Interest Income Recognized	44	37
Impaired loans with related allowance - Interest Income Recognized	3	11
Agriculture production financing and other loans
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	86
Impaired loans with no related allowance - Recorded Investment	86
Impaired loans with no related allowance - Average Recorded Investment	86
Construction
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	6,008		9,787
Impaired loans with related allowance - Unpaid Principal Balance	1,025		1,202
Impaired loans with no related allowance - Recorded Investment	3,928		5,164
Impaired loans with related allowance - Recorded Investment	911		1,058
Impaired loans with related allowance - Related Allowance	85		105
Impaired loans with no related allowance - Average Recorded Investment	3,936	8,994
Impaired loans with related allowance - Average Recorded Investment	915	2,360
Impaired loans with no related allowance - Interest Income Recognized	19	14
Impaired loans with related allowance - Interest Income Recognized
Commercial and farm land
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	52,030		58,173
Impaired loans with related allowance - Unpaid Principal Balance	5,875		5,579
Impaired loans with no related allowance - Recorded Investment	37,803		43,204
Impaired loans with related allowance - Recorded Investment	5,406		5,182
Impaired loans with related allowance - Related Allowance	2,385		2,460
Impaired loans with no related allowance - Average Recorded Investment	39,228	50,333
Impaired loans with related allowance - Average Recorded Investment	5,428	5,821
Impaired loans with no related allowance - Interest Income Recognized	382	289
Impaired loans with related allowance - Interest Income Recognized		36
Residential
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	7,039		8,820
Impaired loans with related allowance - Unpaid Principal Balance	2,725		1,722
Impaired loans with no related allowance - Recorded Investment	4,650		6,215
Impaired loans with related allowance - Recorded Investment	2,390		1,451
Impaired loans with related allowance - Related Allowance	314		50
Impaired loans with no related allowance - Average Recorded Investment	4,737	6,562
Impaired loans with related allowance - Average Recorded Investment	2,390	1,312
Impaired loans with no related allowance - Interest Income Recognized	22	28
Impaired loans with related allowance - Interest Income Recognized
Home Equity
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	4,083		4,199
Impaired loans with no related allowance - Recorded Investment	896		1,006
Impaired loans with no related allowance - Average Recorded Investment	923	305
Impaired loans with related allowance - Average Recorded Investment		215
Impaired loans with no related allowance - Interest Income Recognized	3	3
Impaired loans with related allowance - Interest Income Recognized
Other Loans
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	57		83
Impaired loans with no related allowance - Recorded Investment	9		14
Impaired loans with no related allowance - Average Recorded Investment	10	20
Impaired loans with no related allowance - Interest Income Recognized