Payables to Brokerage Clients - Additional Information (Detail) (USD $) In Billions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Payables to Brokerage Clients Disclosure [Line Items]
Cash balances in interest-bearing brokerage client accounts	$ 32.6	$ 30.6
Average rate paid on cash balances in interest-bearing brokerage client accounts	0.01%	0.01%

The amount of cash received as security in return for loaning securities to another party.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.12)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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