Allowance for Loan Losses - Performing and Nonperforming Loans (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Performing	$ 152,523	$ 156,305
Nonperforming		19
Total	152,523	156,324
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	140,344	145,879
Nonperforming		6
Total	140,344	145,885
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	3,650	1,730
Nonperforming
Total	3,650	1,730
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	8,529	8,696
Nonperforming		13
Total	$ 8,529	$ 8,709