Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	$ 2,707,371,000		$ 2,721,839,000
Financial instrument assets	2,712,820,000		2,898,074,000
Financial instruments sold, not yet purchased, at fair value	2,285,707,000		2,165,500,000
Total fair value of financial instrument liabilities	2,285,707,000		2,165,500,000
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,479,910,000	[1]	2,298,785,000	[2]
Financial instruments sold, not yet purchased, at fair value	2,069,342,000	[1]	1,851,006,000	[2]
Securities netted by respective long and short positions	743,100,000		697,900,000
Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	144,586,000		339,798,000
Financial instruments sold, not yet purchased, at fair value	115,362,000		252,282,000
US Government and Non-US Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	22,983,000		40,053,000
Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	59,832,000	[3]	43,203,000
Financial instruments sold, not yet purchased, at fair value	82,050,000	[3]	42,122,000
Securities netted by respective long and short positions	300,000
Securities on Deposit with Clearing Organizations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities on deposit with clearing organizations			170,235,000	[4]
Investment in CME Group [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	4,435,000	[5]	3,925,000	[5]
Deferred Compensation Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	868,000	[5]	117,000	[5]
Investment In Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	146,000	[5]	1,958,000	[5]
U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	18,953,000		15,076,000
Foreign Currency Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	60,000
Financial instruments sold, not yet purchased, at fair value			5,014,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,707,311,000		2,721,839,000
Financial instrument assets	2,711,746,000		2,895,999,000
Total fair value of financial instrument liabilities	2,285,707,000		2,160,486,000
Level 1 [Member] | Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,479,910,000	[1]	2,298,785,000	[2]
Financial instruments sold, not yet purchased, at fair value	2,069,342,000	[1]	1,851,006,000	[2]
Level 1 [Member] | Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	144,586,000		339,798,000
Financial instruments sold, not yet purchased, at fair value	115,362,000		252,282,000
Level 1 [Member] | US Government and Non-US Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	22,983,000		40,053,000
Level 1 [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	59,832,000	[3]	43,203,000
Financial instruments sold, not yet purchased, at fair value	82,050,000	[3]	42,122,000
Level 1 [Member] | Securities on Deposit with Clearing Organizations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities on deposit with clearing organizations			170,235,000	[4]
Level 1 [Member] | Investment in CME Group [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	4,435,000	[5]	3,925,000	[5]
Level 1 [Member] | Deferred Compensation Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	0	[5]	0	[5]
Level 1 [Member] | Investment In Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	0	[5]	0	[5]
Level 1 [Member] | U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	18,953,000		15,076,000
Level 1 [Member] | Foreign Currency Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0
Financial instruments sold, not yet purchased, at fair value			0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	60,000		0
Financial instrument assets	1,074,000		2,075,000
Total fair value of financial instrument liabilities	0		5,014,000
Level 2 [Member] | Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	[1]	0	[2]
Financial instruments sold, not yet purchased, at fair value	0	[1]	0	[2]
Level 2 [Member] | Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0		0
Financial instruments sold, not yet purchased, at fair value	0		0
Level 2 [Member] | US Government and Non-US Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0		0
Level 2 [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	[3]	0
Financial instruments sold, not yet purchased, at fair value	0	[3]	0
Level 2 [Member] | Securities on Deposit with Clearing Organizations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities on deposit with clearing organizations			0	[4]
Level 2 [Member] | Investment in CME Group [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	0	[5]	0	[5]
Level 2 [Member] | Deferred Compensation Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	868,000	[5]	117,000	[5]
Level 2 [Member] | Investment In Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	146,000	[5]	1,958,000	[5]
Level 2 [Member] | U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	0		0
Level 2 [Member] | Foreign Currency Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	60,000
Financial instruments sold, not yet purchased, at fair value			$ 5,014,000

[1] Equities of $743.1 million have been netted by their respective long and short positions by CUSIP number.

[2] Equities of $697.9 million have been netted by their respective long and short positions by CUSIP number.

[3] Corporate debt instruments of $0.3 million have been netted by their respective long and short positions by CUSIP number.

[4] Securities on deposit with clearing organizations consist of U.S. government obligations and are recorded within Receivable from brokers, dealers and clearing organizations on the Consolidated Statements of Financial Condition.

[5] Investment in CME Group, Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition.