Troubled Debts Restructured (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Investment	Dec. 31, 2011 Investment
Consumer loans | Other
Financing Receivable, Modifications [Line Items]
No. of Loans		2
Post-Modification Outstanding Recorded Balance		$ 48
Residential Mortgage
Financing Receivable, Modifications [Line Items]
No. of Loans		3
Post-Modification Outstanding Recorded Balance		733
Troubled debts restructured | Commercial | Real estate
Financing Receivable, Modifications [Line Items]
No. of Loans	10	6
Pre-Modification Outstanding Recorded Balance	2,239	1,931
Post-Modification Outstanding Recorded Balance	2,399	1,887
Troubled debts restructured | Commercial | Construction and Land Development
Financing Receivable, Modifications [Line Items]
No. of Loans	3	3
Pre-Modification Outstanding Recorded Balance	1,047	4,146
Post-Modification Outstanding Recorded Balance	1,133	4,146
Troubled debts restructured | Commercial | Other
Financing Receivable, Modifications [Line Items]
No. of Loans	6	2
Pre-Modification Outstanding Recorded Balance	325	396
Post-Modification Outstanding Recorded Balance	386	396
Troubled debts restructured | Consumer loans | Real estate
Financing Receivable, Modifications [Line Items]
No. of Loans	28	31
Pre-Modification Outstanding Recorded Balance	972	831
Post-Modification Outstanding Recorded Balance	975	854
Troubled debts restructured | Consumer loans | Other
Financing Receivable, Modifications [Line Items]
No. of Loans	4	1
Pre-Modification Outstanding Recorded Balance	53	14
Post-Modification Outstanding Recorded Balance	52	3
Troubled debts restructured | Consumer loans | Auto
Financing Receivable, Modifications [Line Items]
No. of Loans	4	1
Pre-Modification Outstanding Recorded Balance	34	10
Post-Modification Outstanding Recorded Balance	33	10
Troubled debts restructured | Consumer loans | Boat/RV
Financing Receivable, Modifications [Line Items]
No. of Loans	7	13
Pre-Modification Outstanding Recorded Balance	154	395
Post-Modification Outstanding Recorded Balance	153	376
Troubled debts restructured | Residential Mortgage
Financing Receivable, Modifications [Line Items]
No. of Loans	43	27
Pre-Modification Outstanding Recorded Balance	3,477	2,898
Post-Modification Outstanding Recorded Balance	$ 3,713	$ 3,020