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	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	$ 295,166		$ 97,283
Asset Contract	803,158		244,380
Liabilities Fair Value	239,011		75,390
Liabilities Contract	765,437		260,356
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Foreign Exchange Future [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	21		8
Asset Contract	651		500
Liabilities Fair Value	62		0
Liabilities Contract	244		95
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Equity Future Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	1,442		498
Asset Contract	3,059		1,027
Liabilities Fair Value	595		70
Liabilities Contract	1,046		234
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Equity Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	230		570
Asset Contract	1		1
Liabilities Fair Value	0		0
Liabilities Contract	0		0
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Fixed Income Futures Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	4,183		3,474
Asset Contract	24,400		42,143
Liabilities Fair Value	1,724		5,043
Liabilities Contract	6,601		61,398
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Commodity Contract [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	732		428
Asset Contract	2,278		1,385
Liabilities Fair Value	1,478		410
Liabilities Contract	3,565		1,824
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments designated as hedging instruments [Member] | Foreign currency forward contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	331	[1]
Asset Contract	1	[1]
Liabilities Fair Value	0	[1]
Liabilities Contract	0	[1]
Financial Instruments Owned, at Fair Value [Member] | Derivative instruments not designated as hedging instruments [Member] | Listed equity options [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	288,227		92,305
Asset Contract	772,768		199,324
Liabilities Fair Value	229,721		69,757
Liabilities Contract	753,979		196,804
Financial Instruments Owned, at Fair Value [Member] | Derivative instruments designated as hedging instruments [Member] | Foreign currency forward contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value	0	[1]
Asset Contract	0	[1]
Liabilities Fair Value	5,431	[1]
Liabilities Contract	2	[1]
Accrued Expenses and Other Liabilties [Member] | Derivative instruments not designated as hedging instruments [Member] | Foreign currency forward contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset Fair Value			0
Asset Contract			0
Liabilities Fair Value			110
Liabilities Contract			$ 1

[1] Designated as hedging instrument