Loans - Additional Information (Detail) (USD $)	3 Months Ended								3 Months Ended				3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2010	Mar. 31, 2014 Pass	Dec. 31, 2013 Pass	Mar. 31, 2014 Troubled Debt Restructurings Term Modification One	Dec. 31, 2013 Troubled Debt Restructurings Term Modification One	Mar. 31, 2013 Construction	Mar. 31, 2013 Commercial Real Estate	Mar. 31, 2014 Commercial Real Estate	Dec. 31, 2013 Commercial Real Estate	Mar. 31, 2013 Consumer Loan	Mar. 31, 2014 Commercial	Dec. 31, 2013 Commercial	Mar. 31, 2014 Commercial Pass	Dec. 31, 2013 Commercial Pass	Mar. 31, 2014 Commercial Reclassifications Pass	Mar. 31, 2014 Residential Real Estate	Dec. 31, 2013 Residential Real Estate	Mar. 31, 2014 Residential Real Estate Reclassifications Pass	Mar. 31, 2014 Agriculture	Dec. 31, 2013 Agriculture	Mar. 31, 2014 Agriculture Pass	Dec. 31, 2013 Agriculture Pass	Mar. 31, 2014 Agriculture Reclassifications Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Company's TDRs that were performing according to modified terms	0.00%		63.00%				58.00%	49.00%
Reserves to borrowers whose loan terms have been modified in TDRs outstanding			$ 203,000				$ 2,200,000	$ 2,900,000
Commitment to lend additional amount to borrowers with outstanding loans classified as TDRs							262,000	261,000
TDRs defaulted on restructured loan	0
TDRs defaulted on restructured loan amount									1,600,000	1,500,000			86,000
Past due period over which a default is considered to have occurred			90 days
Minimum outstanding loan balance for credit risk analysis	500,000
Gross loans	$ 682,591,000	$ 709,326,000		$ 1,300,000,000	$ 415,144,000	$ 369,529,000					$ 331,071,000	$ 346,541,000		$ 61,688,000	$ 52,878,000	$ 47,562,000	$ 35,438,000	$ 24,200,000	$ 249,442,000	$ 275,363,000	$ 8,500,000	$ 26,348,000	$ 19,199,000	$ 24,688,000	$ 16,742,000	$ 2,200,000