Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013 Securities	Mar. 31, 2012	Dec. 31, 2012 Securities
Investment Securities [Line Items]
Number of securities in an unrealized loss position	78		57
Percentage of combined depreciation of the combined reported value of the aggregate securities portfolio	2.73%		2.78%
Carrying value of securities pledged	$ 280,880,000		$ 292,880,000
OTTI charges incurred	0	0
Reduced severity rate	15.00%
Severity rate reduction period	7 years
FHLBA Stock
Investment Securities [Line Items]
Investment owned at fair value	10,000,000		11,300,000
FRB Richmond Stock
Investment Securities [Line Items]
Investment owned at fair value	5,580,000		5,580,000
Mortgage-backed securities Non-Agency Alt-A residential
Investment Securities [Line Items]
Voluntary constant prepayment rate	5.00%
Constant default rate scenario	20.00%
Remaining underlying mortgages default period	3 years
Severity rate	71.00%
Equity Securities
Investment Securities [Line Items]
OTTI charges incurred	$ 0	$ 0