Collateralized Transactions and Assets and Liabilities Subject to Netting - Financial Instruments Owned and Pledged to Counterparties that Do Not Have Right to Sell or Repledge (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Repurchase Agreement Counterparty [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge	$ 1,586,700	$ 1,315,803
Right to deliver or repledge [Member]
Repurchase Agreement Counterparty [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge	536,124	552,242
Not having the right to deliver or repledge [Member]
Repurchase Agreement Counterparty [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	$ 979,652	$ 676,956