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Mortgage Banking Activities, Mortgage Servicing Rights (Details) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($)
Summary of Mortgage Servicing Rights [Abstract] |  |  |  |  |
Number of residential mortgage loans serviced for others | 133,538 | 136,528 | 137,359 | 133,538 | 137,359
Outstanding principal balance of residential mortgage loans serviced for others | $ 21,826,773,000 | $ 22,046,632,000 | $ 22,063,121,000 | $ 21,826,773,000 | $ 22,063,121,000
Weighted average interest rate | 3.97% | 3.94% | 3.95% | 3.97% | 3.95%
Remaining term (in months) | 295 months | 297 months | 298 months |  |
Servicing Asset at Fair Value, Amount [Roll Forward] |  |  |  |  |
Beginning balance | $ 278,719,000 | $ 245,858,000 | $ 245,239,000 | $ 252,867,000 | $ 247,073,000
Additions, net | 8,968,000 |  | 9,925,000 | 28,688,000 | 29,439,000
Change in fair value due to principal payments | (8,986,000) |  | (8,667,000) | (25,783,000) | (24,928,000)
Change in fair value due to market assumption changes | 5,972,000 |  | (639,000) | 28,901,000 | (5,726,000)
Ending balance | $ 284,673,000 | $ 252,867,000 | $ 245,858,000 | $ 284,673,000 | $ 245,858,000
Servicing Assets at Fair Value, Assumptions Used to Estimate Fair Value [Abstract] |  |  |  |  |
Discount rate - risk-free rate plus a market premium (in hundredths) | 9.95% | 9.84% | 9.84% |  |
Prepayment rate - based upon loan interest rate, original term and loan type, minimum | 7.85% | 8.72% | 8.71% |  |
Prepayment rate - based upon loan interest rate, original term and loan type, maximum | 15.04% | 15.16% | 15.43% |  |
Loan servicing costs - annually per loan based upon loan type, performing, minimum (in dollars per loan) | $ 66 | $ 65 | $ 65 |  |
Loan servicing costs - annually per loan based upon loan type, performing, maximum (in dollars per loan) | 92 | 88 | 120 |  |
Loan servicing costs - annually per loan based upon loan type, delinquent, minimum (in dollars per loan) | 150 | 150 | 150 |  |
Loan servicing costs - annually per loan based upon loan type, delinquent, maximum (in dollars per loan) | 500 | 500 | 500 |  |
Loan servicing costs - annually per loan based upon loan type, foreclosure, minimum (in dollars per loan) | 1,000 | 1,000 | 1,000 |  |
Loan servicing costs - annually per loan based upon loan type, foreclosure, maximum (in dollars per loan) | $ 4,000 | $ 4,000 | $ 4,000 |  |
Escrow earnings rate - indexed to rates paid on deposit accounts with comparable average life (in hundredths) | 3.07% | 2.24% | 2.00% |  |
Primary secondary mortgage rate spread (in basis points) | 105 | 105 | 105 | 105 | 105

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- Definition Indicates the number of mortgage loans being serviced. + References No definition available. + Details
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- Definition Servicing Assets at Fair Value Assumptions Used to Estimate Fair Value Delinquent Loan Service Cost Minimum + References No definition available. + Details
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- Definition The maximum cost to service a performing loan for the period used as a variable in estimating fair value. + References No definition available. + Details
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- Definition Amount of increase (decrease) in fair value from changes in the assumptions or model used to calculate the fair value of a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. + References No definition available. + Details
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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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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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