Collateralized Transactions - Financial Instruments Owned and Pledged to Counterparties that Do Not Have Right to Sell or Repledge (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Collateralized Agreements [Line Items]
Collateral permitted to be delivered or repledged	$ 5,769,593		$ 4,605,155
Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge (1)	748,084	[1]	696,495	[1]
Assets Within Discontinued Operations [Member]
Collateralized Agreements [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge (1)	16,300
Assets of Business Held For Sale [Member]
Collateralized Agreements [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge (1)			$ 111,500

[1] Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge includes $16.3 million and $111.5 million at June 30, 2013 and December 31, 2012, respectively, recorded within Assets within discontinued operations and Assets of business held for sale, respectively.