Schedule I - CONDENSED FINANCIAL STATEMENTS, STATEMENTS OF FINANCIAL CONDITION (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Aug. 15, 2016	Dec. 31, 2015	Nov. 30, 2015	Dec. 31, 2014	May 31, 2011
ASSETS [Abstract]
Cash and cash equivalents	$ 17,821		$ 17,821	$ 39,812
Investments in securities	36,790		36,790	37,285
Receivable from brokers	1,578		1,578	453
Investment in subsidiaries and receivable from affiliates	5,635		5,635	5,960
Capital lease	2,304		2,304	2,514
Income tax receivable	15,615		15,615	9,349
Other assets	6,066		6,066	6,355
Total assets	128,286		128,286	149,229
LIABILITIES AND EQUITY [Abstract]
Payable to brokers	14,926		14,926	66
Capital lease obligation	4,943		4,943	5,066
Compensation payable	82,907		82,907	42,384
Payable to affiliates	855		855	1,412
Accrued expenses and other liabilities	28,656		28,656	29,178
Sub-total	135,415		135,415	81,921
AC 4% PIK Note (due November 30, 2020)	50,000		50,000	100,000
4.5% Convertible note (net of issuance costs of $138, $165 and $174, respectively) (due August 15, 2021) (Note F)	0		0	109,835
AC 1.6% Note Payable (due February 28, 2018)	15,000		15,000	0
5.875% Senior notes (due June 1, 2021)	24,144		24,144	24,120
Total liabilities	224,559		224,559	315,876
Total equity (deficit)	(96,273)		(96,273)	(166,647)		$ (276,327)		$ 527,795
Total liabilities and equity	$ 128,286		$ 128,286	149,229
AC 4% PIK Note [Member]
LIABILITIES AND EQUITY [Abstract]
Debt instrument, interest rate	4.00%		4.00%				4.00%
Debt instrument, maturity date			Nov. 30, 2020
4.5% Convertible Notes [Member]
LIABILITIES AND EQUITY [Abstract]
Debt instrument, interest rate	4.50%		4.50%		4.50%
Debt instrument, maturity date	Aug. 15, 2021		Aug. 15, 2021
AC 1.6% Note [Member]
LIABILITIES AND EQUITY [Abstract]
AC 1.6% Note Payable (due February 28, 2018)	$ 15,000		$ 15,000
Debt instrument, interest rate	1.60%		1.60%
Debt instrument, maturity date		Feb. 28, 2018	Feb. 28, 2018
5.875% Senior Notes [Member]
LIABILITIES AND EQUITY [Abstract]
Debt instrument, interest rate	5.875%		5.875%						5.875%
Debt instrument, maturity date			Jun. 01, 2021
Parent Company [Member]
ASSETS [Abstract]
Cash and cash equivalents	$ 5,990		$ 5,990	7,209
Investments in securities	36,750		36,750	37,235
Receivable from brokers	1,246		1,246	55
Investment in subsidiaries and receivable from affiliates	108,669		108,669	57,274
Capital lease	2,304		2,304	2,514
Income tax receivable	77,832		77,832	0
Other assets	3,655		3,655	4,149
Total assets	236,446		236,446	108,436
LIABILITIES AND EQUITY [Abstract]
Payable to brokers	14,926		14,926	66
Capital lease obligation	4,943		4,943	5,066
Compensation payable	73,275		73,275	27,496
Securities sold, not yet purchased	0		0	0
Payable to affiliates	134,318		134,318	462
Accrued expenses and other liabilities	16,113		16,113	5,556
Sub-total	243,575		243,575	38,646
AC 4% PIK Note (due November 30, 2020)	50,000		50,000	100,000
4.5% Convertible note (net of issuance costs of $138, $165 and $174, respectively) (due August 15, 2021) (Note F)	0		0	109,835
AC 1.6% Note Payable (due February 28, 2018)	15,000		15,000	0
5.875% Senior notes (due June 1, 2021)	24,144		24,144	24,120
Total liabilities	332,719		332,719	272,601
Total equity (deficit)	(96,273)		(96,273)	(164,165)
Total liabilities and equity	$ 236,446		$ 236,446	$ 108,436
Parent Company [Member] | AC 4% PIK Note [Member]
LIABILITIES AND EQUITY [Abstract]
Debt instrument, interest rate	4.00%		4.00%
Debt instrument, maturity date			Nov. 30, 2020
Parent Company [Member] | 4.5% Convertible Notes [Member]
LIABILITIES AND EQUITY [Abstract]
Debt instrument, interest rate	4.50%		4.50%
Debt instrument, maturity date			Aug. 15, 2021
Parent Company [Member] | AC 1.6% Note [Member]
LIABILITIES AND EQUITY [Abstract]
Debt instrument, interest rate	1.60%		1.60%
Debt instrument, maturity date			Feb. 28, 2018
Parent Company [Member] | 5.875% Senior Notes [Member]
LIABILITIES AND EQUITY [Abstract]
Debt instrument, interest rate	5.875%		5.875%
Debt instrument, maturity date			Jun. 01, 2021