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Securities - Narrative (Detail)
 6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
Security
Dec. 31, 2017
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 9 months 12 days 4 years 8 months 12 days
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 86 68
Total number of securities 91 82
Obligations of states and political subdivisions    
Schedule of Available-for-sale Securities [Line Items]    
Number of securities in an unrealized loss position 58 39
Total number of securities 81 80
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Number of securities in an unrealized loss position 1