Commercial and Retail Credit Exposure Credit Risk Profile by Internal Rating (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 964,847	$ 920,209
Residential Mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans	491,012	434,976
Residential Mortgage | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	474,440	417,772
Residential Mortgage | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	2,031	2,473
Residential Mortgage | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	14,541	14,731
Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Loans	199,776	202,384
Consumer loans | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	98,572	96,864
Consumer loans | Auto
Financing Receivable, Recorded Investment [Line Items]
Loans	15,337	15,203
Consumer loans | Boat/RV
Financing Receivable, Recorded Investment [Line Items]
Loans	79,175	83,557
Consumer loans | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	6,692	6,760
Consumer loans | Pass | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	95,994	94,066
Consumer loans | Pass | Auto
Financing Receivable, Recorded Investment [Line Items]
Loans	15,287	15,135
Consumer loans | Pass | Boat/RV
Financing Receivable, Recorded Investment [Line Items]
Loans	78,012	82,639
Consumer loans | Pass | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	6,595	6,680
Consumer loans | Substandard | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	2,578	2,798
Consumer loans | Substandard | Auto
Financing Receivable, Recorded Investment [Line Items]
Loans	50	68
Consumer loans | Substandard | Boat/RV
Financing Receivable, Recorded Investment [Line Items]
Loans	1,163	918
Consumer loans | Substandard | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	97	80
Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	274,059	282,849
Commercial | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	195,465	197,390
Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	61,876	64,628
Commercial | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans	16,718	20,831
Commercial | Pass | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	169,089	167,991
Commercial | Pass | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	57,472	56,691
Commercial | Pass | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans	8,686	8,093
Commercial | Special Mention | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	15,392	11,940
Commercial | Special Mention | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	300	880
Commercial | Special Mention | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans	172	538
Commercial | Substandard | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	10,984	16,488
Commercial | Substandard | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	3,234	6,260
Commercial | Substandard | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans	7,860	12,105
Commercial | Doubtful | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans		971
Commercial | Doubtful | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	870	797
Commercial | Doubtful | Construction and development
Financing Receivable, Recorded Investment [Line Items]
Loans		$ 95