Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Cash flows from operating activities:
Net income (loss)	$ 83,217	$ (37,091)	$ (12,395)
Adjustments to reconcile net income (loss) to net cash provided by / (used in) operating activities:
Bargain purchase gain, net of tax	0	(6,914)	0
Depreciation and amortization	12,436	13,078	12,713
Gain (Loss) on Disposition of Business	0	0	(15,638)
Amortization of debt issuance costs	1,153	1,261	1,208
Amortization of debt discount	5,473	7,435	6,885
Gain / (loss) on extinguishment of debt	652	10,551	0
Excess Tax Benefit from Share-based Compensation, Operating Activities	0	0	(822)
Share-based compensation	37,018	28,443	26,039
Change in deferred taxes	15,486	45,856	(21,274)
Deferred rent obligations	(3,036)	(2,192)	(880)
Net loss (gain) on disposal of fixed assets	15,343	(2,346)	0
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	0	0	2,139
Purchases of securities owned, at fair value	(3,830,757)	(4,943,394)	(4,483,975)
Proceeds from sales of securities owned, at fair value	4,032,937	5,131,938	4,447,094
Proceeds from sales of securities sold, not yet purchased, at fair value	2,794,434	2,455,876	3,016,934
Payments to cover securities sold, not yet purchased, at fair value	(2,927,916)	(2,444,010)	(3,054,869)
Proceeds from the sale of other investments	10,157	0	0
Net (gains) losses on securities, derivatives and other investments	(54,032)	(78,303)	(28,878)
Consolidated Funds
Purchases of securities owned, at fair value	(887,728)	(385,544)	(113,653)
Proceeds from sales of securities owned, at fair value	835,002	338,268	73,011
Proceeds from sales of securities sold, not yet purchased, at fair value	0	217	2,226
Payments to cover securities sold, not yet purchased, at fair value	0	(899)	(1,799)
Purchases of other investments	(2,835)	(26,206)	(221,897)
Proceeds from sales of other investments	231,559	64,867	17,116
Net realized and unrealized (gains) losses on investments and other transactions	(52,807)	(28,267)	(17,402)
(Increase) decrease in operating assets:
Cash acquired through acquisition	0	159,587	0
Securities owned, at fair value, held at broker-dealer	(57,732)	(102,137)	(17,005)
Receivable on derivative contracts, at fair value	44,053	(46,277)	16,717
Securities borrowed	35,353	(184,827)	0
Deposits with clearing organizations, brokers and banks	4,573	(32,460)	(1,874)
Receivable from brokers, dealers and clearing organizations	(277,935)	(366,297)	31,795
Receivable from customers, net of allowance	12,033	(6,422)	0
Fees receivable, net of allowance	(6,401)	(29,669)	(12,632)
Due from related parties	1,405	4,815	(176)
Other assets	6,017	(33,454)	(10,048)
Consolidated Funds
Cash and cash equivalents	(19,501)	(4,227)	(3,827)
Receivable on derivative contracts, at fair value	(1,896)	(1,627)	(893)
Receivable from brokers	(2,684)	334	(5,978)
Other assets	(538)	123	152
Increase (decrease) in operating liabilities:
Securities sold, not yet purchased, at fair value, held at broker dealer	26,232	19,226	7,160
Payable for derivative contracts, at fair value	2,305	(1,399)	(421)
Securities loaned	(41,979)	170,472	0
Payable to brokers, dealers and clearing organizations	(23,422)	(32,440)	78,520
Payable to customers	172,686	340,666	0
Commission management payable	24,819	(15,857)	242
Compensation payable	61,478	9,150	(60,279)
Fees payable	14,518	(639)	(2,366)
Due to related parties	1	(3)	244
Accounts payable, accrued expenses and other liabilities	17,723	13,046	(9,133)
Consolidated Funds
Contributions received in advance	0	(2,000)	1,150
Payable to brokers	22,770	(2,949)	3,699
Payable for derivative contracts, at fair value	(5,467)	6,558	572
Due to related parties	0	(189)	453
Accounts payable, accrued expenses and other liabilities	369	(330)	528
Net Cash Provided by (Used in) Operating Activities	324,536	3,398	(351,517)
Cash flows from investing activities:
Purchases of other investments	(28,092)	(10,419)	(33,786)
Purchase of business	0	(55,049)	(6,258)
Proceeds from sales of other investments	18,782	71,749	54,068
Payments to Acquire Loans Receivable	0	(13,745)
Proceeds from Divestiture of Businesses, Net of Cash Divested	0	0	17,303
Proceeds from loans held for investment	14	3,203	42,800
Purchase of fixed assets	(8,586)	(5,986)	(15,613)
Sale of fixed assets	0	7,850	0
Net Cash Provided by (Used in) Investing Activities	(17,882)	(2,397)	58,514
Cash flows from financing activities:
Proceeds from Convertible Debt	0	135,000	0
Repayments on convertible debt	(13,500)	(115,140)	0
Deferred debt issuance cost	(3,985)	(7,176)	0
Borrowings on notes and other debt	102,382	171,148	30,638
Repayments on notes and other debt	(16,601)	(72,757)	(29,800)
Excess Tax Benefit from Share-based Compensation, Financing Activities	0	0	(822)
Purchase of treasury stock	(31,762)	(19,662)	(7,654)
Contingent liability payment	(797)	(393)	(2,358)
Capital contributions by redeemable non-controlling interests in operating entities	581	0	10
Capital withdrawals to redeemable non-controlling interests in operating entities	(2,774)	(5,178)	(6,995)
Consolidated Funds
Capital contributions by redeemable non-controlling interests in Consolidated Funds	114,064	117,812	276,914
Capital withdrawals to redeemable non-controlling interests in Consolidated Funds	(276,357)	(65,803)	(10,144)
Net Cash Provided by (Used in) Financing Activities	(128,749)	137,851	249,789
Change in cash and cash equivalents	177,905	138,852	(43,214)
Total cash beginning of period	264,208	125,356	168,570
Cash and cash equivalents	259,148	130,052	110,990
Cash collateral pledged	6,318	17,888	13,342
Segregated cash	176,647	116,268	1,024
Total cash at end of period	442,113	264,208	125,356
Supplemental information
Cash paid during the year for interest	90,843	26,632	17,540
Cash paid during the year for taxes	3,881	2,172	5,085
Noncash Conversion Difference Between Discount and Derivative Liability	0	23,387	0
Purchase of treasury stock, at cost, through net settlement	12,310	11,889	8,835
Net assets (liabilities) acquired upon acquisition (net of cash)	0	77,790	0
Transfer of investment from consolidated funds, securities owned, fair value to securities owned, fair value	8,820	0	0
Net decrease in redeemable non-controlling interests in Consolidated Funds due to deconsolidation of consolidated fund	32,559	0	73,042
Separately recognized conversion option reclassification from a derivative liability to equity	21,195	0	0
Common stock issuance upon close of acquisition	0	47,607	0
Transfer from Investments	0	0	7,164
Conversion of redeemable non-controlling interest to loan receivable	0	1,299	$ 0
Goodwill, Impairment Loss	$ 0	$ 0