Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Commercial | Real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	$ 4,200	$ 5,932	$ 5,549
Unpaid Principal Balance	5,264	7,282	7,301
Specific Allowance	100	144	100
Average Investment in Impaired Loans	4,344	6,500	5,595
Interest Income Recognized	44	13	260
Commercial | Construction and development
Financing Receivable, Impaired [Line Items]
Recorded Balance	7,932	11,287	8,271
Unpaid Principal Balance	12,721	16,433	12,235
Specific Allowance	1,159	1,437	959
Average Investment in Impaired Loans	8,101	12,115	10,114
Interest Income Recognized	14	10	108
Commercial | Other
Financing Receivable, Impaired [Line Items]
Recorded Balance	1,570	4,762	1,884
Unpaid Principal Balance	1,570	4,762	1,884
Specific Allowance	380	543	257
Average Investment in Impaired Loans	1,727	5,079	3,749
Interest Income Recognized	9	2	147
Residential Mortgage
Financing Receivable, Impaired [Line Items]
Recorded Balance	3,174	4,425	3,417
Unpaid Principal Balance	4,113	5,916	4,356
Specific Allowance	57	37	57
Average Investment in Impaired Loans	3,296	5,170	4,409
Interest Income Recognized	22	22	58
Loans without a specific valuation allowance | Commercial | Real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	3,992	4,840	5,341
Unpaid Principal Balance	5,056	5,690	6,354
Average Investment in Impaired Loans	4,136	5,158	5,384
Interest Income Recognized	41	13	248
Loans without a specific valuation allowance | Commercial | Construction and development
Financing Receivable, Impaired [Line Items]
Recorded Balance	2,667	4,216	3,632
Unpaid Principal Balance	4,842	9,152	7,078
Average Investment in Impaired Loans	2,804	5,044	4,884
Interest Income Recognized	8	10	30
Loans without a specific valuation allowance | Commercial | Other
Financing Receivable, Impaired [Line Items]
Recorded Balance	936	3,292	972
Unpaid Principal Balance	936	3,292	972
Average Investment in Impaired Loans	954	3,609	2,828
Interest Income Recognized	3	2	117
Loans without a specific valuation allowance | Residential Mortgage
Financing Receivable, Impaired [Line Items]
Recorded Balance	2,341	3,892	2,583
Unpaid Principal Balance	3,280	5,383	3,522
Average Investment in Impaired Loans	2,463	4,635	3,755
Interest Income Recognized	13	22	58
Loans with a specific valuation allowance | Commercial | Real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	208	1,092	208
Unpaid Principal Balance	208	1,592	947
Specific Allowance	100	144	100
Average Investment in Impaired Loans	208	1,342	211
Interest Income Recognized	3		12
Loans with a specific valuation allowance | Commercial | Construction and development
Financing Receivable, Impaired [Line Items]
Recorded Balance	5,265	7,071	4,639
Unpaid Principal Balance	7,879	7,281	5,157
Specific Allowance	1,159	1,437	959
Average Investment in Impaired Loans	5,297	7,071	5,230
Interest Income Recognized	6		78
Loans with a specific valuation allowance | Commercial | Other
Financing Receivable, Impaired [Line Items]
Recorded Balance	634	1,470	912
Unpaid Principal Balance	634	1,470	912
Specific Allowance	380	543	257
Average Investment in Impaired Loans	773	1,470	921
Interest Income Recognized	6		30
Loans with a specific valuation allowance | Residential Mortgage
Financing Receivable, Impaired [Line Items]
Recorded Balance	833	533	834
Unpaid Principal Balance	833	533	834
Specific Allowance	57	37	57
Average Investment in Impaired Loans	833	535	654
Interest Income Recognized	$ 9