Commercial and Retail Credit Exposure Credit Risk Profile by Internal Rating (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 975,557	$ 990,569
Residential Mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans	491,133	502,619
Residential Mortgage | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	474,762	486,027
Residential Mortgage | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	1,991	2,012
Residential Mortgage | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	14,380	14,580
Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Loans	197,041	199,102
Consumer loans | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	100,461	100,516
Consumer loans | Auto
Financing Receivable, Recorded Investment [Line Items]
Loans	15,082	15,572
Consumer loans | Boat/RV
Financing Receivable, Recorded Investment [Line Items]
Loans	75,205	76,416
Consumer loans | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	6,293	6,598
Consumer loans | Pass | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	98,107	97,972
Consumer loans | Pass | Auto
Financing Receivable, Recorded Investment [Line Items]
Loans	15,050	15,533
Consumer loans | Pass | Boat/RV
Financing Receivable, Recorded Investment [Line Items]
Loans	73,723	75,026
Consumer loans | Pass | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	6,152	6,434
Consumer loans | Substandard | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	2,354	2,544
Consumer loans | Substandard | Auto
Financing Receivable, Recorded Investment [Line Items]
Loans	32	39
Consumer loans | Substandard | Boat/RV
Financing Receivable, Recorded Investment [Line Items]
Loans	1,482	1,390
Consumer loans | Substandard | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	141	164
Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	287,383	288,848
Commercial | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	202,046	203,613
Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	68,276	67,773
Commercial | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans	17,061	17,462
Commercial | Pass | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	185,917	185,794
Commercial | Pass | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	65,626	63,413
Commercial | Pass | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans	9,320	9,314
Commercial | Special Mention | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	6,383	6,692
Commercial | Special Mention | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	202	255
Commercial | Special Mention | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans	150	172
Commercial | Substandard | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	9,746	11,127
Commercial | Substandard | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	1,838	3,281
Commercial | Substandard | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans	7,591	7,976
Commercial | Doubtful | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 610	$ 824