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Securities - Additional Information (Detail)
 9 Months Ended 12 Months Ended
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