Commitments (Activity in Reserve for Representation and Warranty Provisions) (Detail) - Residential Mortgage - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Valuation And Qualifying Accounts Disclosure
Balance, beginning of period	$ 13	$ 25
Net (reductions) additions to the reserve	(3)	(10)
Losses charged against the reserve	(1)	(2)
Balance, end of period	$ 9	$ 13

Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-09)
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690

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Total of the deductions in a given period to allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs, representing receivables written off as uncollectible and portions of the reserves utilized, respectively.

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Amount of increase (decrease) in allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690

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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.

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No definition available.

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