Consolidated Statements of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income (loss)	$ 182,780,000	$ 17,840,000	$ (23,957,000)
Adjustments to reconcile net income (loss) to net cash provided by / (used in) operating activities:
Depreciation and amortization	10,188,000	10,227,000	9,437,000
Amortization of Debt Discount	4,685,000	0	0
Income tax effect from share based compensation	1,324,000	0	0
Share-based compensation	18,297,000	17,915,000	24,784,000
Deferred tax benefit	(130,724,000)	0	0
Deferred rent obligations	(2,348,000)	(3,770,000)	(1,938,000)
Net loss on disposal of fixed assets	1,575,000	410,000	30,000
Gain (Loss) on Sale of Capital Leases, Net	(1,261,000)	0	0
Goodwill impairment	2,334,000	0	0
Contingent liability adjustment	(2,055,000)	0	0
Purchases of securities owned, at fair value	(4,722,554,000)	(6,765,271,000)	(6,129,663,000)
Proceeds from sales of securities owned, at fair value	4,272,785,000	7,057,506,000	6,204,613,000
Proceeds from sales of securities sold, not yet purchased, at fair value	1,992,965,000	2,740,372,000	4,253,982,000
Payments to cover securities sold, not yet purchased, at fair value	(1,898,102,000)	(2,774,696,000)	(4,388,039,000)
Net (gains) losses on securities, derivatives and other investments	(97,013,000)	(44,741,000)	(43,443,000)
Consolidated Funds
Purchases of securities owned, at fair value	0	(298,220,000)	(366,388,000)
Proceeds from sales of securities owned, at fair value	0	287,412,000	369,209,000
Proceeds from sales of securities sold, not yet purchased, at fair value	0	42,701,000	0
Payments to cover securities sold, not yet purchased, at fair value	0	(42,387,000)	0
Purchases of other investments	(19,736,000)	(4,497,000)	(9,785,000)
Proceeds from sales of other investments	34,225,000	56,873,000	42,071,000
Net realized and unrealized (gains) losses on investments and other transactions	(16,386,000)	(14,153,000)	(8,608,000)
(Increase) decrease in operating assets:
Cash acquired upon transaction	0	10,747,000	290,000
Cash at deconsolidated entity	(784,000)	0	0
Cash collateral pledged	2,601,000	135,000	625,000
Securities owned, at fair value, held at broker dealer	(3,939,000)	20,080,000	34,632,000
Receivable on derivative contracts, at fair value	(4,092,000)	10,673,000	35,828,000
Securities borrowed	251,673,000	(521,447,000)	121,528,000
Receivable from brokers	(15,533,000)	5,218,000	(1,365,000)
Fees receivable, net of allowance	(1,406,000)	(9,975,000)	(5,642,000)
Due from related parties	(109,000)	(5,888,000)	(3,570,000)
Other assets	(15,925,000)	2,818,000	10,056,000
Consolidated Funds
Cash and cash equivalents	1,547,000	1,511,000	(2,326,000)
Other assets	4,187,000	(5,528,000)	1,370,000
Increase (decrease) in operating liabilities:
Securities sold, not yet purchased, at fair value, held at broker dealer	(21,633,000)	(10,994,000)	8,322,000
Payable for derivative contracts, at fair value	(2,055,000)	(2,374,000)	(34,356,000)
Securities loaned	(236,084,000)	509,605,000	(134,397,000)
Payable to brokers	258,587,000	(113,023,000)	(23,102,000)
Compensation payable	78,264,000	(13,036,000)	(29,047,000)
Fees payable	(129,000)	1,047,000	(985,000)
Due to related parties	92,000	(280,000)	(1,252,000)
Accounts payable, accrued expenses and other liabilities	7,375,000	(13,404,000)	(12,954,000)
Consolidated Funds
Payable to brokers	0	1,030,000	0
Due To related parties	0	0	25,000
Accounts payable, accrued expenses and other liabilities	(327,000)	146,000	149,000
Net cash provided by / (used in) operating activities	(66,711,000)	150,582,000	(103,866,000)
Cash flows from investing activities:
Securities purchased under agreement to resell	0	0	166,260,000
Purchases of other investments	(81,597,000)	(20,031,000)	(14,848,000)
Cash convertible note economic hedge transaction	(35,710,000)	0	0
Proceeds from sales of other investments	82,072,000	29,890,000	13,298,000
Purchase of business, net of cash acquired	0	0	(10,853,000)
Purchase of fixed assets	(2,224,000)	(1,141,000)	(1,902,000)
Net cash provided by / (used in) investing activities	(37,459,000)	8,718,000	151,955,000
Cash flows from financing activities:
Securities sold under agreement to repurchase	(3,657,000)	(162,288,000)	(62,838,000)
Proceeds from issuance of convertible debt	149,500,000	0	0
Deferred debt issuance cost	(3,720,000)	0	0
Proceeds from sale of warrant	15,218,000	0	0
Borrowings on notes and other debt	65,392,000	2,044,000	0
Repayments on notes and other debt	(3,627,000)	(3,613,000)	(1,518,000)
Excess Tax Benefit from Share-based Compensation, Financing Activities	1,324,000	0	0
Proceeds from stock options exercised	116,000	0	0
Purchase of treasury stock	(26,038,000)	(12,732,000)	(10,838,000)
Cash paid to acquire net assets (contingent liability payable)	(800,000)	(779,000)	0
Capital contributions by redeemable non-controlling interests in operating entities	705,000	501,000	0
Capital withdrawals to redeemable non-controlling interests in operating entities	(8,279,000)	(3,058,000)	(3,167,000)
Consolidated Funds
Capital contributions by redeemable non-controlling interests in Consolidated Funds	9,736,000	14,680,000	0
Capital withdrawals to redeemable non-controlling interests in Consolidated Funds	(16,911,000)	(22,873,000)	(15,065,000)
Net cash provided by / (used in) financing activities	178,959,000	(188,118,000)	(93,426,000)
Change in cash and cash equivalents	74,789,000	(28,818,000)	(45,337,000)
Cash and cash equivalents at beginning of period	54,720,000	83,538,000	128,875,000
Cash and cash equivalents at end of period	129,509,000	54,720,000	83,538,000
Supplemental information
Cash paid during the year for interest	32,032,000	24,644,000	9,419,000
Cash paid during the year for taxes	547,000	72,000	611,000
Net assets (liabilities) acquired upon acquisition (net of cash)	0	0	9,995,000
Non compete agreements and covenants with limiting conditions acquired	0	460,000	167,000
Common stock issuance upon close of acquisition	0	6,240,000	0
Purchase of treasury stock, at cost, through net settlement	5,649,000	3,649,000	3,988,000
Cash conversion option	35,710,000	0	0
Assets acquired under capital lease	4,075,000	0	0
Re-issuance of treasury stock for services provided	0	65,000	0
Net assets transferred to Merger Master	0	22,152,000	0
Net assets of consolidated entities	0	0	18,521,000
Net assets of deconsolidated entities	$ 1,544,000	$ 0	$ 17,104,000