Activity in Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 28,124	$ 56,680
Provision for loan losses		450
Loans charged off	(3,082)	(17,962)
Recoveries	373	671
Ending balance	25,415	39,839
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	3,221	4,402
Provision for loan losses	445	1,438
Loans charged off	(146)	(976)
Recoveries	88	126
Ending balance	3,608	4,990
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	16,414	34,768
Provision for loan losses	(1,127)	(445)
Loans charged off	(1,474)	(12,312)
Recoveries	116	158
Ending balance	13,929	22,169
Residential Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	7,762	16,235
Provision for loan losses	534	(450)
Loans charged off	(1,308)	(4,339)
Recoveries	83	94
Ending balance	7,071	11,540
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	416	857
Provision for loan losses	19	86
Loans charged off	(128)	(318)
Recoveries	76	91
Ending balance	383	716
Agriculture
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	305	403
Provision for loan losses	113	(178)
Loans charged off	(9)	(17)
Recoveries	6	202
Ending balance	415	410
Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6	15
Provision for loan losses	16	(1)
Loans charged off	(17)
Recoveries	4
Ending balance	$ 9	$ 14