Loans and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	$ 32,463,000	$ 27,447,000	$ 37,724,000
Recorded Investment With No Allowance	6,154,000	2,083,000	7,290,000
Recorded Investment With Allowance	18,556,000	20,426,000	20,636,000
Total Recorded Investment	24,710,000	22,509,000	27,926,000
Related Allowance	5,283,000	5,793,000	5,338,000
Average Recorded Investment	26,770,000	23,163,000	31,293,000
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	3,250,000	1,555,000	4,764,000
Recorded Investment With No Allowance	258,000	25,000	934,000
Recorded Investment With Allowance	2,551,000	1,108,000	3,348,000
Total Recorded Investment	2,809,000	1,133,000	4,282,000
Related Allowance	1,200,000	555,000	1,079,000
Average Recorded Investment	2,956,000	1,232,000	5,017,000
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	123,000	489,000	139,000
Recorded Investment With No Allowance	15,000	25,000	17,000
Recorded Investment With Allowance	94,000	448,000	114,000
Total Recorded Investment	109,000	473,000	131,000
Related Allowance	36,000	169,000	41,000
Average Recorded Investment	111,000	483,000	144,000
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	28,028,000	24,937,000	31,704,000
Recorded Investment With No Allowance	5,387,000	1,956,000	5,794,000
Recorded Investment With Allowance	15,534,000	18,546,000	16,753,000
Total Recorded Investment	20,921,000	20,502,000	22,547,000
Related Allowance	3,865,000	4,901,000	4,006,000
Average Recorded Investment	22,764,000	21,028,000	25,060,000
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	1,062,000	466,000	1,117,000
Recorded Investment With No Allowance	494,000	77,000	545,000
Recorded Investment With Allowance	377,000	324,000	421,000
Total Recorded Investment	871,000	401,000	966,000
Related Allowance	182,000	168,000	212,000
Average Recorded Investment	$ 939,000	$ 420,000	$ 1,072,000