Certain Regulatory Requirements and Capital Ratios - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Average deposited in a special reserve account for the exclusive benefit of customers pursuant to SEC Regulations	$ 1,500	$ 744
Trust Preferred Securities [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier I capital (to risk-weighted assets)	$ 810
TruPS as percentage of risk-weighted assets	74

Tier 1 Risk Based Capital as defined in the regulations.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=7881181&loc=d3e65071-112826

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

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 15

-Subparagraph 3a

-IssueDate 2006-05-01

-Chapter 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958568-112826

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