Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income (loss)	$ 55,870	$ 79,879	$ (37,091)
Adjustments to reconcile net income (loss) to net cash provided by / (used in) operating activities:
Depreciation and amortization	20,460	12,436	13,078
Amortization of debt issuance costs	1,129	1,153	1,261
Amortization of debt discount	4,598	5,473	7,435
Noncash lease expense	(2,148)	0	0
Gain / (loss) on extinguishment of debt	0	(652)	(10,551)
Share-based compensation	33,918	37,018	28,443
Change in deferred taxes	13,295	15,486	45,856
Deferred rent obligations	0	(3,036)	(2,192)
Net loss (gain) on disposal of fixed assets	(233)	(15,343)	2,346
Goodwill Impairment	4,100	0	0
Purchases of securities owned, at fair value	(2,107,956)	(3,830,757)	(4,943,394)
Proceeds from sales of securities owned, at fair value	2,042,946	4,032,937	5,131,938
Proceeds from sales of securities sold, not yet purchased, at fair value	1,360,075	2,794,434	2,455,876
Payments to cover securities sold, not yet purchased, at fair value	(1,408,043)	(2,927,916)	(2,444,010)
Proceeds from other investments	23,886	10,157	0
Net (gains) losses on securities, derivatives and other investments	(74,284)	(54,032)	(78,303)
Consolidated Funds
Purchases of securities owned, at fair value	(2,725,439)	(887,728)	(385,544)
Proceeds from sales of securities owned, at fair value	2,618,200	835,002	338,268
Purchases of other investments	(3,408)	(2,835)	(26,206)
Proceeds from other investments	23,954	231,559	64,867
Net realized and unrealized (gains) losses on investments and other transactions	(90,900)	(52,807)	(28,267)
(Increase) decrease in operating assets:
Securities owned, at fair value, held at broker-dealer	(990,356)	(57,732)	(102,137)
Receivable on derivative contracts, at fair value	(37,522)	44,053	(46,277)
Securities borrowed	(346,646)	35,353	(184,827)
Deposits with clearing organizations, brokers and banks	(2,332)	4,573	(32,460)
Receivable from brokers, dealers and clearing organizations	104,418	(277,935)	(366,297)
Receivable from customers, net of allowance	(67,789)	12,033	(6,422)
Fees receivable, net of allowance	(7,143)	(6,401)	(29,669)
Due from related parties	7,120	1,405	4,815
Other assets	(6,501)	6,017	(33,454)
Consolidated Funds
Cash and cash equivalents	7,299	(19,501)	(4,227)
Receivable on derivative contracts, at fair value	(1,417)	(1,896)	(1,627)
Receivable from brokers	(17,636)	(2,684)	334
Other assets	(159)	(538)	123
Increase (decrease) in operating liabilities:
Securities sold, not yet purchased, at fair value, held at broker dealer	277,119	26,232	19,226
Payable for derivative contracts, at fair value	44,679	2,305	(1,399)
Securities loaned	1,187,014	(41,979)	170,472
Payable to brokers, dealers and clearing organizations	42,287	(23,422)	(32,440)
Payable to customers	(94,929)	172,686	340,666
Commission management payable	(23,650)	24,819	(15,857)
Compensation payable	(38,954)	64,456	9,150
Fees payable	(1,025)	14,518	(639)
Due to related parties	(5,320)	1	(3)
Accounts payable, accrued expenses and other liabilities	(3,148)	17,723	13,046
Increase (Decrease) in contributions received in advance, Consolidated Funds	50	0	(2,000)
Consolidated Funds
Payable to brokers	(22,657)	22,770	(2,949)
Payable for derivative contracts, at fair value	3,106	(5,467)	6,558
Due To Related Parties, Consolidated Funds	581	0	(189)
Accounts payable, accrued expenses and other liabilities	(129)	369	(330)
Net Cash Provided by (Used in) Operating Activities	(179,910)	324,176	3,398
Cash flows from investing activities:
Purchases of other investments	(19,812)	(28,092)	(10,419)
Purchase of business	(48,581)	0	(55,049)
Proceeds from sales of other investments	35,648	18,782	71,749
Payments to Acquire Loans Receivable	0	0	13,745
Proceeds from loans held for investment	0	14	3,203
Purchase of fixed assets	(14,882)	(8,586)	(5,986)
Proceeds from Sale of Property, Plant, and Equipment	0	0	7,850
Net Cash Provided by (Used in) Investing Activities	(47,627)	(17,882)	(2,397)
Proceeds from Convertible Debt	0	0	135,000
Cash flows from financing activities:
Proceeds from (Payments for) in Securities Sold under Agreements to Repurchase	23,244	0	0
Repayments on convertible debt	(20,860)	(13,500)	(115,140)
Deferred debt issuance cost	(2,077)	(3,985)	(7,176)
Borrowings on notes and other debt	87,365	102,382	171,148
Repayments on notes and other debt	(10,263)	(16,601)
Purchase of treasury stock	(15,217)	(31,762)	(19,662)
Contingent liability payment	(1,234)	(797)	(393)
Capital contributions by redeemable non-controlling interests in operating entities	11,110	941	0
Capital withdrawals to redeemable non-controlling interests in operating entities	(3,785)	(2,774)	(5,178)
Consolidated Funds
Capital contributions by redeemable non-controlling interests in Consolidated Funds	267,049	114,064	117,812
Capital withdrawals to redeemable non-controlling interests in Consolidated Funds	(111,650)	(276,357)	(65,803)
Net Cash Provided by (Used in) Financing Activities	200,438	(128,389)	137,851
Change in cash and cash equivalents	(27,099)	177,905	138,852
Total cash beginning of period	442,113	264,208	125,356
Cash and cash equivalents	301,123	259,148	130,052
Cash collateral pledged	6,563	6,318	17,888
Segregated cash	107,328	176,647	116,268
Total cash at end of period	415,014	442,113	264,208
Supplemental information
Cash paid during the year for interest	145,877	90,843	26,632
Cash paid during the year for taxes	4,310	3,881	2,172
Purchase of treasury stock, at cost, through net settlement	15,217	12,310	11,889
Preferred stock dividends	6,792	6,792	6,792
Net assets (liabilities) acquired upon acquisition (net of cash)	90,727	0	77,790
Operating right of use assets	92,852	0
Operating lease liabilities	97,581	0
Transfer of investment from consolidated funds, securities owned, fair value to securities owned, fair value	97,655	8,820	0
Net decrease in redeemable non-controlling interests in Consolidated Funds due to deconsolidation of consolidated fund	0	32,559
Separately recognized conversion option reclassification from a derivative liability to equity	(596)	28,974
Common stock issuance upon close of acquisition	14,446	0	47,607
ProceedsFromSecuritiesSoldNotYetPurchasedAtFairValue,ConsolidatedFunds	0	0	217
Payments to Acquire Securities Sold Not Yet Purchased at Fair Value, Consolidated Funds	0	0	899
Cash Acquired from Acquisition	$ 0	$ 0	159,587
Excess Tax Benefit from Share-based Compensation, Financing Activities			$ (72,757)