Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Cash Flows [Abstract]
Cash and cash equivalents	$ 117,680	$ 77,456
Cash flows from operating activities:
Net income (loss)	28,009	12,089
Adjustments to reconcile net income (loss) to net cash provided by / (used in) operating activities:
Depreciation and amortization	3,225	3,028
Amortization of debt issuance costs	322	304
Amortization of debt discount	1,442	1,840
Share-based compensation	10,241	8,779
Deferred tax benefit	7,115	1,782
Deferred rent obligations	(888)	(225)
Purchases of securities owned, at fair value	(1,943,349)	(1,215,330)
Proceeds from sales of securities owned, at fair value	2,060,263	1,324,115
Proceeds from sales of securities sold, not yet purchased, at fair value	1,276,205	710,012
Payments to cover securities sold, not yet purchased, at fair value	(1,386,253)	(673,243)
Net (gains) losses on securities, derivatives and other investments	(7,924)	(33,493)
Consolidated Funds
Purchases of securities owned, at fair value	(148,548)	(63,563)
Proceeds from sales of securities owned, at fair value	175,010	55,704
Proceeds from sales of securities sold, not yet purchased, at fair value	0	217
Payments to cover securities sold, not yet purchased, at fair value	0	(899)
Purchases of other investments	(81)	(8,578)
Proceeds from sales of other investments	4,774	14,788
Net realized and unrealized (gains) losses on investments and other transactions	(18,304)	(11,138)
(Increase) decrease in operating assets:
Cash acquired through acquisition	0	0
Securities owned, at fair value, held at broker-dealer	13,197	(30,255)
Receivable on derivative contracts, at fair value	34,265	4,764
Securities borrowed	(154,942)	0
Deposits with clearing organizations, brokers and banks	(7,403)	(4,048)
Receivable from brokers, dealers and clearing organizations	(54,163)	(71,737)
Receivable from customers, net of allowance	3,925	0
Fees receivable, net of allowance	(30,996)	(4,042)
Due from related parties	4,874	(3,163)
Other assets	8,062	(20,199)
Consolidated Funds
Cash and cash equivalents	18,322	7,468
Receivable on derivative contracts, at fair value	654	(16)
Receivable from brokers	1,731	(3,311)
Other assets	(58)	(7)
Increase (decrease) in operating liabilities:
Securities sold, not yet purchased, at fair value, held at broker dealer	40,622	22,772
Payable for derivative contracts, at fair value	6,125	(6,619)
Securities loaned	6,023	0
Payable to brokers, dealers and clearing organizations	(4,532)	(85,858)
Payable to customers	60,179	70,480
Commission management payable	9,354	1,229
Compensation payable	(69,441)	(70,745)
Fees payable	11,183	4,604
Due to related parties	(26)	0
Accounts payable, accrued expenses and other liabilities	13,193	13,234
Consolidated Funds
Contributions received in advance	670	(2,048)
Payable to brokers	865	(1,950)
Payable for derivative contracts, at fair value	(5,150)	304
Due to related parties	0	(189)
Accounts payable, accrued expenses and other liabilities	73	(211)
Net cash provided by / (used in) operating activities	(35,506)	(53,354)
Cash flows from investing activities:
Purchases of other investments	(11,475)	(4,934)
Proceeds from sales of other investments	10,271	16,180
Loans issued	(3,050)	0
Proceeds from loans held for investment	10	1,200
Purchase of fixed assets	(1,151)	(150)
Net cash provided by / (used in) investing activities	(5,395)	12,296
Cash flows from financing activities:
Deferred debt issuance costs	(104)	0
Borrowings on notes and other debt	88,061	2,247
Repayments on notes and other debt	(1,475)	(1,504)
Purchase of treasury stock	(7,584)	0
Cash paid to acquire net assets (contingent liability payment)	0	(167)
Capital contributions by redeemable non-controlling interests in operating entities	200	0
Capital withdrawals to redeemable non-controlling interests in operating entities	(513)	(2,059)
Consolidated Funds
Capital contributions by redeemable non-controlling interests in Consolidated Funds	8,059	23,198
Capital withdrawals to redeemable non-controlling interests in Consolidated Funds	(46,041)	(12,357)
Net cash provided by / (used in) financing activities	40,603	9,358
Change in cash and cash equivalents	(298)	(31,700)
Total cash beginning of period	264,208	125,355
Total cash at end of period	263,910	93,655
Supplemental information
Purchase of treasury stock, at cost, through net settlement (See Note 18)	4,910	4,455
Preferred stock dividends declared (See Note 18)	1,698	1,698
Restricted Cash and Cash Equivalents	14,786	14,696
Segregated cash	$ 131,444	$ 1,503