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Securities - Additional Information (Detail)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
Security
Sep. 30, 2016
Security
Sep. 30, 2017
USD ($)
Security
Sep. 30, 2016
USD ($)
Security
Dec. 31, 2016
Security
Schedule of Available-for-sale Securities [Line Items]          
Percentage of investment portfolio     100.00%   100.00%
Weighted-average period of re-pricing of portfolio     4 years 3 months 12 days   4 years 8 months 12 days
Securities sold from the held to maturity portfolio 0 0 0 1  
Proceeds from sale | $       $ 657,000  
Impairment recognized | $     $ 0    
U.S. agency and mortgage-backed securities          
Schedule of Available-for-sale Securities [Line Items]          
Number of securities in an unrealized loss position 50   50   64
Total number of securities 86   86   83
Obligations of states and political subdivisions          
Schedule of Available-for-sale Securities [Line Items]          
Number of securities in an unrealized loss position 11   11   50
Total number of securities 82   82   78
Corporate debt securities          
Schedule of Available-for-sale Securities [Line Items]          
Number of securities in an unrealized loss position 1   1   1