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Investment Securities - Additional Information (Detail) (USD $) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2013 Securities | Sep. 30, 2012 | Sep. 30, 2013 Securities | Sep. 30, 2012 | Dec. 31, 2012 Securities
Investment Securities [Line Items] |  |  |  |  |
Number of securities in an unrealized loss position | 171 |  | 171 |  | 57
Percentage of combined depreciation of the combined reported value of the aggregate securities portfolio | 4.43% |  | 4.43% |  | 2.78%
Carrying value of securities pledged | $ 279,880,000 |  | $ 279,880,000 |  | $ 292,880,000
OTTI charges incurred | 0 | 942,000 | 0 | 942,000 |
Reduced severity rate | 15.00% |  | 15.00% |  |
Severity rate reduction period | 6 years |  | 6 years |  |
Mortgage-backed securities Non-Agency Alt-A residential |  |  |  |  |
Investment Securities [Line Items] |  |  |  |  |
Voluntary constant prepayment rate | 9.70% |  | 9.70% |  |
Constant default rate scenario | 12.00% |  | 12.00% |  |
Remaining underlying mortgages default period | 3 years |  | 3 years |  |
Severity rate | 66.00% |  | 66.00% |  |
Equity Securities |  |  |  |  |
Investment Securities [Line Items] |  |  |  |  |
OTTI charges incurred | $ 0 | $ 0 | $ 0 | $ 0 |

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