Commitments (Activity in Reserve for Representation and Warranty Provisions) (Detail) - Residential Mortgage - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Valuation And Qualifying Accounts Disclosure
Balance, beginning of period	$ 7	$ 11	$ 9	$ 13
Net (reductions) additions to the reserve	(1)	(1)	(3)	(3)
Balance, end of period	$ 6	$ 10	$ 6	$ 10

Amount of increase (decrease) in valuation and qualifying accounts and reserves.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-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

Name:	us-gaap_ValuationAllowancesAndReservesPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration