Key Assumptions Used in Calculating Fair Value of Residential Mortgage Servicing Rights (Detail) - $ / LoanPerYear	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Expected weighted average life	87 months	82 months
Constant prepayment speed	9.81%	8.70%
Discount rate	12.00%	10.05%
Primary mortgage rate to refinance	4.02%	4.11%
Cost to service per loan per year, current	70	64
30-59 days or less delinquent
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	220	214
60-89 days delinquent
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	370	364
90-119 days delinquent
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	470	464
120 days or more delinquent
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	870	864

Cost To Service Loan Per Loan Per Year

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- Definition

Cost To Service Loan Per Loan Per Year Current

+ References

No definition available.

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- Definition

Discount rate which is used to value residual cash flows generated by financial assets of a securitization, asset-backed financing arrangement, or similar transfer regardless of when the transfer occurred.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

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- Definition

The stated interest rate on the mortgage loan receivable or the weighted average interest rate on a group of loans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29)
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule IV)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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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

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- Definition

Weighted average life of financial assets and financial liabilities, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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