Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Cash flows from operating activities:
Net income (loss)	$ 30,006	$ 28,585	$ 58,015	$ 40,674
Adjustments to reconcile net income (loss) to net cash provided by / (used in) operating activities:
Bargain purchase gain, net of tax			0	(7,946)
Depreciation and amortization	3,194	3,132	6,419	6,160
Amortization of debt issuance costs			656	608
Amortization of debt discount			2,871	3,699
Gain / (loss) on extinguishment of debt			(652)	0
Share-based compensation			19,080	17,905
Deferred tax benefit			10,953	5,201
Deferred rent obligations			(939)	(175)
Net loss (gain) on disposal of fixed assets			155	55
Purchases of securities owned, at fair value			(2,525,789)	(2,432,174)
Proceeds from sales of securities owned, at fair value			2,779,569	2,628,293
Proceeds from sales of securities sold, not yet purchased, at fair value			1,789,059	1,316,612
Payments to cover securities sold, not yet purchased, at fair value			(1,940,907)	(1,430,940)
Proceeds from the sale of other investments			3,870	0
Net (gains) losses on securities, derivatives and other investments			(28,237)	(50,070)
Consolidated Funds
Purchases of securities owned, at fair value			(306,995)	(137,781)
Proceeds from sales of securities owned, at fair value			344,079	130,550
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			(1,414)	(8,628)
Proceeds from sales of other investments			10,289	31,478
Net realized and unrealized (gains) losses on investments and other transactions			(47,628)	(34,282)
(Increase) decrease in operating assets:
Cash acquired through acquisition			0	159,587
Securities owned, at fair value, held at broker-dealer			(89,077)	(131,681)
Receivable on derivative contracts, at fair value			43,982	(7,928)
Securities borrowed			(104,973)	157,476
Deposits with clearing organizations, brokers and banks			(660)	(26,612)
Receivable from brokers, dealers and clearing organizations			(159,702)	5,435
Receivable from customers, net of allowance			(5,321)	(20,726)
Fees receivable, net of allowance			(32,917)	(34,215)
Due from related parties			2,448	(623)
Other assets			13,027	(25,591)
Consolidated Funds
Cash and cash equivalents			5,590	(16,646)
Receivable on derivative contracts, at fair value			882	(122)
Receivable from brokers			(3,294)	(3,732)
Other assets			(91)	159
Increase (decrease) in operating liabilities:
Securities sold, not yet purchased, at fair value, held at broker dealer			30,292	77,783
Payable for derivative contracts, at fair value			10,097	5,179
Securities loaned			(22,365)	(143,925)
Payable to brokers, dealers and clearing organizations			(11,510)	(16,659)
Payable to customers			168,099	102,648
Commission management payable			21,721	6,176
Compensation payable			(28,644)	(60,243)
Fees payable			15,562	1,999
Due to related parties			4	1
Accounts payable, accrued expenses and other liabilities			6,747	22,578
Consolidated Funds
Contributions received in advance			0	(2,000)
Payable to brokers			1,241	4,569
Payable for derivative contracts, at fair value			(6,085)	48
Due to related parties			0	(189)
Accounts payable, accrued expenses and other liabilities			225	(312)
Net cash provided by / (used in) operating activities	29,036		29,036	130,991
Cash flows from investing activities:
Purchases of other investments			(20,227)	(60,367)
Purchase of business			0	(54,017)
Proceeds from sales of other investments			13,295	99,127
Proceeds from loans held for investment			13	2,400
Purchase of fixed assets			(1,397)	(2,175)
Net cash provided by / (used in) investing activities	(8,316)		(8,316)	(15,032)
Cash flows from financing activities:
Repayments on convertible debt			(13,500)	0
Deferred debt issuance cost			(3,883)	0
Borrowings on notes and other debt			105,516	31,737
Repayments on notes and other debt			(2,987)	(2,740)
Purchase of treasury stock			(10,862)	(11,279)
Contingent liability payment			(570)	0
Cash paid to acquire net assets (contingent liability payment)			0	(267)
Capital contributions by redeemable non-controlling interests in operating entities			200	0
Capital withdrawals to redeemable non-controlling interests in operating entities			(1,113)	(3,778)
Consolidated Funds
Capital contributions by redeemable non-controlling interests in Consolidated Funds			18,937	35,149
Capital withdrawals to redeemable non-controlling interests in Consolidated Funds			(58,136)	(22,045)
Net cash provided by / (used in) financing activities			33,602	26,777
Change in cash and cash equivalents			54,322	142,736
Total cash beginning of period			264,208	125,356	$ 125,356
Cash and cash equivalents	207,708	101,058	207,708	101,058	130,052
Cash collateral pledged	15,513	15,473	15,513	15,473	17,888
Segregated cash	95,309	151,561	95,309	151,561	116,268
Total cash at end of period	$ 318,530	$ 268,092	318,530	268,092	$ 264,208
Supplemental information
Interest Paid			39,061	9,231
Income Taxes Paid			2,233	1,535
Purchase of treasury stock, at cost, through net settlement			8,376	11,279
Preferred stock dividends declared			3,396	3,396
Noncash or Part Noncash Acquisition, Net Nonmonetary Assets Acquired (Liabilities Assumed)			0	(50,017)
Transfer to Investments			8,820	0
Noncontrolling Interest, Decrease from Deconsolidation			32,559	0
Embedded Cash Conversion Option			28,973	0
Common stock issued during period, value, acquisitions			$ 0	$ 47,607