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Mortgage Servicing Rights - Key Assumptions and the Sensitivity of the Fair Value (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Transfers and Servicing [Abstract] |  |
Aggregate portfolio principal balance | $ 439,561 | $ 406,291
Weighted average rate of note | 3.77% | 3.77%
Conditional prepayment rate | 8.20% | 9.00%
Impact of 1 percent adverse change in prepayment speed, percent | 21.33% | 23.33%
Impact of 2 percent adverse change in prepayment speed, percent | 23.12% | 25.25%
Discount rate | 9.73% | 9.45%
Impact of 1 percent adverse change in discount rate, percent | 8.73% | 8.45%
Impact of 2 percent adverse change in discount rate, percent | 7.73% | 7.45%
Mortgage servicing rights | $ 8,039 | $ 7,305
Impact of 1 percent adverse change, fair value of mortgage servicing rights | 4,895 | 4,343
Impact of 2 percent adverse change, fair value of mortgage servicing rights | $ 4,648 | $ 4,109
MSR as a percentage of serviced loans | 1.83% | 1.80%
Impact of 1 percent adverse change, percentage of mortgage servicing rights | 1.11% | 1.07%
Impact of 2 percent adverse change, percentage of mortgage servicing rights | 1.06% | 1.01%

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- Definition Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements. + 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)(1) -URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746 + Details
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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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- Definition Estimated rate of prepayments of principal on 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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