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Servicing - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Transfers and Servicing [Abstract] |  |
Loans serviced for others unpaid principal amount | $ 288.6 | $ 280.4
Capitalized service right assets included in other assets | 2.3 | 2.2
Fair market value of the capitalized servicing rights | $ 3.1 | $ 2.5
Prepayment rate | 10.40% | 14.30%
Discount yield | 6.07% | 5.38%

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- Definition Discount rate used to estimate the fair value of servicing assets and servicing liabilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746 + Details
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