Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Commercial | Real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	$ 3,570		$ 3,570		$ 6,966
Unpaid Principal Balance	4,122		4,122		7,764
Specific Allowance					209
Average Investment in Impaired Loans					5,307
Average Investment in Impaired Loans	4,546	4,989	5,646	6,538
Interest Income Recognized	37	12	147	133	211
Commercial | Construction and development
Financing Receivable, Impaired [Line Items]
Recorded Balance	8,475		8,475		12,943
Unpaid Principal Balance	13,438		13,438		19,082
Specific Allowance	375		375		1,437
Average Investment in Impaired Loans					13,226
Average Investment in Impaired Loans	9,033	13,143	10,574	15,898
Interest Income Recognized	29	55	90	287	581
Commercial | Other
Financing Receivable, Impaired [Line Items]
Recorded Balance	2,254		2,254		5,500
Unpaid Principal Balance	2,254		2,254		5,691
Specific Allowance	357		357		543
Average Investment in Impaired Loans					1,726
Average Investment in Impaired Loans	3,289	1,483	4,234	2,222
Interest Income Recognized	14	4	63	35	271
Residential Mortgage
Financing Receivable, Impaired [Line Items]
Recorded Balance	4,489		4,489		5,915
Unpaid Principal Balance	5,577		5,577		7,407
Specific Allowance	37		37		36
Average Investment in Impaired Loans					5,599
Average Investment in Impaired Loans	4,143	4,765	4,656	5,866
Interest Income Recognized	37	47	93	119	178
Loans without a specific valuation allowance | Commercial | Real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	3,570		3,570		4,883
Unpaid Principal Balance	4,122		4,122		5,275
Average Investment in Impaired Loans					4,221
Average Investment in Impaired Loans	4,546	3,349	5,646	4,890
Interest Income Recognized	37	4	147	81	137
Loans without a specific valuation allowance | Commercial | Construction and development
Financing Receivable, Impaired [Line Items]
Recorded Balance	7,111		7,111		5,872
Unpaid Principal Balance	9,101		9,101		11,801
Average Investment in Impaired Loans					9,451
Average Investment in Impaired Loans	7,669	9,158	9,092	11,776
Interest Income Recognized	11	31	35	227	348
Loans without a specific valuation allowance | Commercial | Other
Financing Receivable, Impaired [Line Items]
Recorded Balance	1,076		1,076		4,030
Unpaid Principal Balance	1,076		1,076		4,167
Average Investment in Impaired Loans					1,480
Average Investment in Impaired Loans	2,098	1,358	3,028	1,972
Interest Income Recognized	5	1	28	28	211
Loans without a specific valuation allowance | Residential Mortgage
Financing Receivable, Impaired [Line Items]
Recorded Balance	3,960		3,960		5,378
Unpaid Principal Balance	5,048		5,048		6,870
Average Investment in Impaired Loans					5,532
Average Investment in Impaired Loans	3,613	4,765	4,124	5,866
Interest Income Recognized	19	47	58	119	142
Loans with a specific valuation allowance | Commercial | Real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance					2,083
Unpaid Principal Balance					2,489
Specific Allowance					209
Average Investment in Impaired Loans					1,086
Average Investment in Impaired Loans		1,640		1,648
Interest Income Recognized		8		52	74
Loans with a specific valuation allowance | Commercial | Construction and development
Financing Receivable, Impaired [Line Items]
Recorded Balance	1,364		1,364		7,071
Unpaid Principal Balance	4,337		4,337		7,281
Specific Allowance	375		375		1,437
Average Investment in Impaired Loans					3,775
Average Investment in Impaired Loans	1,364	3,985	1,482	4,122
Interest Income Recognized	18	24	55	60	233
Loans with a specific valuation allowance | Commercial | Other
Financing Receivable, Impaired [Line Items]
Recorded Balance	1,178		1,178		1,470
Unpaid Principal Balance	1,178		1,178		1,524
Specific Allowance	357		357		543
Average Investment in Impaired Loans					246
Average Investment in Impaired Loans	1,191	125	1,206	250
Interest Income Recognized	9	3	35	7	60
Loans with a specific valuation allowance | Residential Mortgage
Financing Receivable, Impaired [Line Items]
Recorded Balance	529		529		537
Unpaid Principal Balance	529		529		537
Specific Allowance	37		37		36
Average Investment in Impaired Loans					67
Average Investment in Impaired Loans	530		532
Interest Income Recognized	$ 18		$ 35		$ 36