Asset Quality - Nonperforming Assets and Past Due Loans (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Receivables [Abstract]
Purchased credit impaired loans	$ 13	$ 16
Restructured loans	$ 17	$ 13

Amount of restructured debt contingently payable based on the occurrence of future events or circumstances.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 60

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6402221&loc=d3e15765-112638

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