Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	$ 2,523,977		$ 654,875
Total fair value of financial instrument assets	8,705,198		861,654
Financial instruments sold, not yet purchased, at fair value	2,162,938		512,553
Total fair value of financial instrument liabilities	7,966,797		512,553
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,156,466	[1]	378,933	[2]
Financial instruments sold, not yet purchased, at fair value	1,848,729		423,740	[2]
Listed equity options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	288,227		92,305
Financial instruments sold, not yet purchased, at fair value	229,721		69,757
Corporate debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	46,606		68,765
Financial instruments sold, not yet purchased, at fair value	49,616		19,056
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	160,369	[3]	7,147
Deferred compensation investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	84	[4]
Investment in Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument assets	1,430	[4]	199,632	[5]
Securitized HECM loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	5,841,664	[6]
Loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	177,674		114,872
U.S. government obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	32,678
Financial instruments sold, not yet purchased, at fair value	29,441
Foreign currency forward contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	5,431
Liability to GNMA trusts, at fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities	5,803,859	[6]
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,523,977		654,875
Total fair value of financial instrument assets	2,684,346		861,654
Financial instruments sold, not yet purchased, at fair value	2,157,507		512,553
Total fair value of financial instrument liabilities	2,157,507		512,553
Level 1 [Member] | Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,156,466	[1]	378,933	[2]
Financial instruments sold, not yet purchased, at fair value	1,848,729	[1]	423,740	[2]
Level 1 [Member] | Listed equity options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	288,227		92,305
Financial instruments sold, not yet purchased, at fair value	229,721		69,757
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	46,606		68,765
Financial instruments sold, not yet purchased, at fair value	49,616		19,056
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	160,369	[3]	7,147
Level 1 [Member] | Deferred compensation investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	0	[4]
Level 1 [Member] | Investment in Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument assets	0	[4]	199,632	[5]
Level 1 [Member] | Securitized HECM loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	[6]
Level 1 [Member] | Loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0		114,872
Level 1 [Member] | U.S. government obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	32,678
Financial instruments sold, not yet purchased, at fair value	29,441
Level 1 [Member] | Foreign currency forward contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	0
Level 1 [Member] | Liability to GNMA trusts, at fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities	0	[6]
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0		0
Total fair value of financial instrument assets	6,020,852		0
Financial instruments sold, not yet purchased, at fair value	5,431		0
Total fair value of financial instrument liabilities	5,809,290		0
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 equity 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] | Corporate debt [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] | 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	[3]
Level 2 [Member] | Deferred compensation investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	84	[4]
Level 2 [Member] | Investment in Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument assets	1,430	[4]		[5]
Level 2 [Member] | Securitized HECM loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	5,841,664	[6]
Level 2 [Member] | Loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	177,674
Level 2 [Member] | U.S. government obligations [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] | Foreign currency forward contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	5,431
Level 2 [Member] | Liability to GNMA trusts, at fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities	5,803,859	[6]
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0		0
Total fair value of financial instrument assets	0		0
Financial instruments sold, not yet purchased, at fair value	0		0
Total fair value of financial instrument liabilities	0		0
Level 3 [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 3 [Member] | Listed equity 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 3 [Member] | Corporate debt [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 3 [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	[3]
Level 3 [Member] | Deferred compensation investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	0	[4]
Level 3 [Member] | Investment in Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument assets	0	[4]	0	[5]
Level 3 [Member] | Securitized HECM loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	[6]
Level 3 [Member] | Loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0		0
Level 3 [Member] | U.S. government obligations [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 3 [Member] | Foreign currency forward contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	0
Level 3 [Member] | Liability to GNMA trusts, at fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities	$ 0	[6]

[1] Equities of $1.22 billion have been netted by their respective CUSIP number and their long and short positions.

[2] Equities of $5.9 million have been netted by their respective CUSIP number and their long and short positions.

[3] 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.

[4] Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition

[5]

[6] Represents HECMs that have been securitized into HECM Mortgage Backed Securities (“HMBS”) where the securitization is not accounted for as a sale of the underlying HECMs. See Securitized HECM loan inventory below for full description.