Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	$ 7,395,982		$ 6,024,018
Total fair value of financial instrument assets	7,573,743		6,370,668
Financial instruments sold, not yet purchased, at fair value	1,512,983		1,445,437
Total fair value of financial instrument liabilities	6,797,286		5,647,308
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	1,609,957	[1]	1,463,916	[2]
Financial instruments sold, not yet purchased, at fair value	1,290,401	[1]	1,164,999	[2]
U.S. government obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			34,339
Financial instruments sold, not yet purchased, at fair value	33,532		71,728
U.S. government and Non-U.S. government obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	24,006
Corporate debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	114,724		76,818	[3]
Financial instruments sold, not yet purchased, at fair value	56,889		47,263	[3]
Listed equity options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	188,166		202,091
Financial instruments sold, not yet purchased, at fair value	132,161		155,942
Loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	130,635		191,712
Purchased call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			237
Securitized HECM loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	5,327,418	[4]	4,054,905	[5]
Securities segregated under federal and other regulations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities segregated under federal and other regulations			3,000	[6]
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	155,203	[7]	182,280	[6]
Deferred compensation investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	21,231	[8]	21,339	[9]
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	1,327	[8]	1,342	[9]
Embedded conversion derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value			237
Foreign currency forward contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	1,076
Financial instruments sold, not yet purchased, at fair value			5,268
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	5,284,303	[4]	4,002,704	[5]
Liabilities Held for Sale - Corporate Deb [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities			22,858
Liabilities Held for Sale U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities			176,309
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	1,936,853		1,777,164
Total fair value of financial instrument assets	2,092,056		1,992,134
Financial instruments sold, not yet purchased, at fair value	1,512,983		1,439,932
Total fair value of financial instrument liabilities	1,512,983		1,639,099
Level 1 [Member] | Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	1,609,957	[1]	1,463,916	[2]
Financial instruments sold, not yet purchased, at fair value	1,290,401	[1]	1,164,999	[2]
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			34,339
Financial instruments sold, not yet purchased, at fair value	33,532		71,728
Level 1 [Member] | U.S. government and Non-U.S. government obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	24,006
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	114,724		76,818	[3]
Financial instruments sold, not yet purchased, at fair value	56,889		47,263	[3]
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	188,166		202,091
Financial instruments sold, not yet purchased, at fair value	132,161		155,942
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		0
Level 1 [Member] | Purchased call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			0
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	[4]	0	[5]
Level 1 [Member] | Securities segregated under federal and other regulations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities segregated under federal and other regulations			3,000	[6]
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	155,203	[7]	182,280	[6]
Level 1 [Member] | Deferred compensation investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	0	[8]	0	[9]
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	[8]	0	[9]
Level 1 [Member] | Embedded conversion derivative [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] | 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 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	[4]	0	[5]
Level 1 [Member] | Liabilities Held for Sale - Corporate Deb [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities			22,858
Level 1 [Member] | Liabilities Held for Sale U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities			176,309
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	5,459,129		4,246,854
Total fair value of financial instrument assets	5,481,687		4,378,534
Financial instruments sold, not yet purchased, at fair value	0		5,505
Total fair value of financial instrument liabilities	5,284,303		4,008,209
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] | 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		0
Level 2 [Member] | U.S. government and Non-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
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	[3]
Financial instruments sold, not yet purchased, at fair value	0		0	[3]
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] | Loan inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	130,635		191,712
Level 2 [Member] | Purchased call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			237
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	5,327,418	[4]	4,054,905	[5]
Level 2 [Member] | Securities segregated under federal and other regulations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities segregated under federal and other regulations			0	[6]
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	[7]	0	[6]
Level 2 [Member] | Deferred compensation investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	21,231	[8]	21,339	[9]
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	1,327	[8]	1,342	[9]
Level 2 [Member] | Embedded conversion derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value			237
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	1,076
Financial instruments sold, not yet purchased, at fair value			5,268
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	5,284,303	[4]	4,002,704	[5]
Level 2 [Member] | Liabilities Held for Sale - Corporate Deb [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities			0
Level 2 [Member] | Liabilities Held for Sale U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities			0
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] | 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		0
Level 3 [Member] | U.S. government and Non-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
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	[3]
Financial instruments sold, not yet purchased, at fair value	0		0	[3]
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] | 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] | Purchased call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			0
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	[4]	0	[5]
Level 3 [Member] | Securities segregated under federal and other regulations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities segregated under federal and other regulations			0	[6]
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	[7]	0	[6]
Level 3 [Member] | Deferred compensation investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	0	[8]	0	[9]
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	[8]	0	[9]
Level 3 [Member] | Embedded conversion derivative [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] | 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	[4]	0	[5]
Level 3 [Member] | Liabilities Held for Sale - Corporate Deb [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities			0
Level 3 [Member] | Liabilities Held for Sale U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of financial instrument liabilities			0
Assets Held-for-sale [Member] | Corporate debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			29,690
Assets Held-for-sale [Member] | MBS held within Assets of business held for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			108,999
Assets Held-for-sale [Member] | 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			29,690
Assets Held-for-sale [Member] | Level 1 [Member] | MBS held within Assets of business held for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			0
Assets Held-for-sale [Member] | 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
Assets Held-for-sale [Member] | Level 2 [Member] | MBS held within Assets of business held for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			108,999
Assets Held-for-sale [Member] | 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
Assets Held-for-sale [Member] | Level 3 [Member] | MBS held within Assets of business held for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value			$ 0

[1] Equities of $621.0 million have been netted by their respective CUSIP number and their long and short positions.

[2] Equities of $676.8 million have been netted by their respective CUSIP number and their long and short positions.

[3] Corporate debt of $0.1 million has been netted by respective CUSIP number and their long and short positions.

[4] 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.

[5] Represents HECMs that have been securitized into HMBS where the securitization is not accounted for as a sale of the underlying HECMs. See Securitized HECM loan inventory below for full description.

[6] Securities segregated under federal and other regulations and Securities on deposit with clearing organizations consist of U.S. government obligations. The securities on deposit with clearing organizations are recorded within Receivable from brokers, dealers and clearing organizations on the Consolidated Statements of Financial Condition.

[7] 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.

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

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