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Investment Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Securities	Dec. 31, 2011 Securities
Investment Securities [Line Items]
Carrying value of securities pledged	$ 292,880,000	$ 288,800,000
Number of held-to-maturity securities in a continuous unrealized loss position	12	12
Number of securities in an unrealized loss position	57	28
Percentage of combined depreciation of the combined reported value of the aggregate securities portfolio	2.78%	4.51%
OTTI charges incurred	942,000	2,290,000
Reduced severity rate	15.00%
Severity rate reduction period	7 years
Mortgage-backed securities Non-Agency Alt-A residential
Investment Securities [Line Items]
Voluntary constant prepayment rate	5.00%
Constant default rate scenario	21.00%
Remaining underlying mortgages default period	3 years
Severity rate	72.00%
Equity Securities
Investment Securities [Line Items]
OTTI charges incurred	$ 0	$ 0