MSR (Sales of Receivables and Servicing Rights - Additional Information) (Detail) - USD ($) $ in Billions	Mar. 31, 2018	Dec. 31, 2017
Disclosure - Activity Related to Mortgage Banking Net Revenue [Abstract]
Sevicing of residential mortgage loans for other investors	$ 61.0	$ 60.0

This is the principal amount outstanding for securitized loans only (across all types of loans).

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

Name:	us-gaap_PrincipalAmountOutstandingOnLoansSecuritized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant