Fair Value of Financial Instruments - Fair Value of Derivative Instruments in Consolidated Statements of Financial Condition and Effect of Changes in Fair Value on Consolidated Statements of Operations (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments owned, at fair value	$ 7,395,982		$ 6,024,018
Financial instruments sold, not yet purchased, at fair value	1,512,983		1,445,437
Derivative instruments not designated as hedging instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments owned, at fair value	188,166		202,328
Financial instruments sold, not yet purchased, at fair value	132,161		157,001
Derivative instruments not designated as hedging instruments [Member] | Purchased call options [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments owned, at fair value	0		237
Derivative instruments not designated as hedging instruments [Member] | Listed equity options [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments owned, at fair value	188,166	[1]	202,091	[1]
Financial instruments sold, not yet purchased, at fair value	132,161	[1]	155,942	[1]
Derivative instruments not designated as hedging instruments [Member] | Foreign currency forward contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments sold, not yet purchased, at fair value	0		822
Derivative instruments not designated as hedging instruments [Member] | Embedded conversion derivative [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments sold, not yet purchased, at fair value	0		237
Derivative instruments designated as hedging instruments [Member] | Foreign currency forward contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments owned, at fair value	1,076		0
Financial instruments sold, not yet purchased, at fair value	$ 0		$ 4,446

[1] As of June 30, 2013, the Company held 0.6 million each of long and short listed equity option contracts. As of December 31, 2012, the Company held 0.9 million long and 1.0 million short listed equity option contracts. These contracts are not subject to collateral requirements and are not netted.