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Disclosures about Fair Value of Assets and Liabilities - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Fair Value Disclosures [Abstract] |  |
Reduced in carrying amount of mortgage servicing rights | $ 587,000 | $ 507,000

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- Definition The increase (decrease) in the fair value of servicing assets that are subsequently measured at fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746 + Details
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