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Mortgage Servicing Rights, Net - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Mortgage Servicing Rights [Line Items] |  |
Primary servicing portfolio | $ 55,100.0 | $ 54,200.0
Special servicing portfolio | $ 3,600.0 | $ 3,800.0
Minimum [Member] |  |
Mortgage Servicing Rights [Line Items] |  |
Discount rate | 3.00% | 3.00%
Maximum [Member] |  |
Mortgage Servicing Rights [Line Items] |  |
Discount rate | 13.50% | 13.50%
Discount Rate One [Member] |  |
Mortgage Servicing Rights [Line Items] |  |
Increase in discount rate | 1.00% | 1.00%
Decrease in fair value of servicing rights | $ (13.7) | $ (11.8)
Discount Rate Two [Member] |  |
Mortgage Servicing Rights [Line Items] |  |
Increase in discount rate | 2.00% | 2.00%
Decrease in fair value of servicing rights | $ (25.0) | $ (23.0)
Mortgage Servicing Rights [Member] |  |
Mortgage Servicing Rights [Line Items] |  |
Estimated fair value of MSRs | $ 422.2 | $ 418.1

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- Definition Percentage of increase in discount rate of servicing rights. + References No definition available. + Details
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- Definition Primary servicing portfolio. + References No definition available. + Details
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- Definition Special servicing portfolio. + References No definition available. + Details
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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + 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/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 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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