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Commitments and Letters of Credit - Additional Information (Detail) - USD ($) | Dec. 31, 2017 | Dec. 31, 2016
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Loss Contingencies [Line Items] |  |
Notional amount of financial standby letters of credit | $ 1,500,000 |
Notional amount of performance standby letters of credit | 5,100,000 |
Notional amount of commercial letters of credit | 313,000 |
Financial instrument of credit risk | 198,000 | $ 200,000
Unused commitments and letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Commitment liability | $ 5,200,000 | $ 4,100,000

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