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Securities - Additional Information (Detail) | 9 Months Ended | 12 Months Ended
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Sep. 30, 2015 USD ($) Securities Obligation | Dec. 31, 2014 Securities Obligation
Amortized Cost and Fair Value Debt Securities [Abstract] |  |
Number of U.S. agency and mortgage-backed securities | Securities | 31 | 29
Number of obligations of state and political subdivisions in an unrealized loss position | Obligation | 20 | 7
Percentage of investment portfolio | 100.00% | 100.00%
Weighted-average period of re-pricing of portfolio | 4 years 4 months 24 days | 3 years 10 months 24 days
Impairment recognized | $ 0 |

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