Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Parenthetical) (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Residential Non-Agency Mortgage-Backed Securities Unrealized Losses	$ 218
Non-Agency [Member] \| Mortgage-Backed Securities Residential [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Residential Non-Agency Mortgage-Backed Securities Unrealized Losses	$ 4

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant