Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Cash flows from operating activities:
Net income (loss)	$ 20,778	$ 9,828	$ 78,793	$ 50,502
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	3,139	3,452	9,558	9,612
Amortization of debt issuance costs			990	936
Amortization of debt discount			4,177	5,601
Gain / (loss) on extinguishment of debt			(652)	0
Share-based compensation			27,636	24,702
Deferred tax benefit			14,640	7,039
Deferred rent obligations			(2,532)	(1,503)
Net loss (gain) on disposal of fixed assets			146	(2,362)
Purchases of securities owned, at fair value			(3,137,168)	(3,442,431)
Proceeds from sales of securities owned, at fair value			3,329,707	3,620,347
Proceeds from sales of securities sold, not yet purchased, at fair value			2,132,935	1,809,863
Payments to cover securities sold, not yet purchased, at fair value			(2,233,427)	(1,799,136)
Proceeds from the sale of other investments			7,313	0
Net (gains) losses on securities, derivatives and other investments			(61,927)	(76,325)
Consolidated Funds
Purchases of securities owned, at fair value			(457,293)	(261,863)
Proceeds from sales of securities owned, at fair value			491,427	241,307
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)	(26,206)
Proceeds from sales of other investments			230,832	51,709
Net realized and unrealized (gains) losses on investments and other transactions			(58,523)	(43,423)
(Increase) decrease in operating assets:
Cash acquired through acquisition			0	159,587
Securities owned, at fair value, held at broker-dealer			(124,740)	(58,126)
Receivable on derivative contracts, at fair value			36,949	(9,204)
Securities borrowed			(270,140)	(113,046)
Deposits with clearing organizations, brokers and banks			(4,650)	(22,625)
Receivable from brokers, dealers and clearing organizations			(136,214)	(105,255)
Receivable from customers, net of allowance			16,651	(2,040)
Fees receivable, net of allowance			(6,211)	(28,839)
Due from related parties			2,765	2,222
Other assets			16,573	(31,415)
Consolidated Funds
Cash and cash equivalents			(130)	6,605
Receivable on derivative contracts, at fair value			609	(149)
Receivable from brokers			(5,373)	(2,356)
Other assets			(153)	222
Increase (decrease) in operating liabilities:
Securities sold, not yet purchased, at fair value, held at broker dealer			146,853	(100)
Payable for derivative contracts, at fair value			16,225	5,555
Securities loaned			91,798	128,598
Payable to brokers, dealers and clearing organizations			(41,827)	(128,490)
Payable to customers			118,009	190,926
Commission management payable			19,782	(8,457)
Compensation payable			30,500	(38,092)
Fees payable			14,211	2,807
Due to related parties			1	(6)
Accounts payable, accrued expenses and other liabilities			1,527	19,109
Consolidated Funds
Contributions received in advance			0	(2,000)
Payable to brokers			1,185	(2,289)
Payable for derivative contracts, at fair value			(4,941)	1,175
Due to related parties			0	(189)
Accounts payable, accrued expenses and other liabilities			205	(438)
Net cash provided by / (used in) operating activities			295,986
Net Cash Provided by (Used in) Operating Activities			295,986	123,431
Cash flows from investing activities:
Purchases of other investments			(21,355)	(72,834)
Purchase of business			0	(54,017)
Proceeds from sales of other investments			15,730	118,130
Proceeds from loans held for investment			1,410	3,200
Purchase of fixed assets			(6,710)	(3,056)
Sale of fixed assets			0	7,850
Net Cash Provided by (Used in) Investing Activities			(10,925)	(727)
Net cash provided by / (used in) investing activities			(10,925)
Cash flows from financing activities:
Repayments on convertible debt			(13,500)	0
Deferred debt issuance cost			(3,985)	0
Borrowings on notes and other debt			102,382	31,737
Repayments on notes and other debt			(6,561)	(8,845)
Purchase of treasury stock			(16,763)	0
Contingent liability payment			(797)	(393)
Capital contributions by redeemable non-controlling interests in operating entities			581	0
Capital withdrawals to redeemable non-controlling interests in operating entities			(1,685)	(4,628)
Consolidated Funds
Capital contributions by redeemable non-controlling interests in Consolidated Funds			24,594	60,322
Capital withdrawals to redeemable non-controlling interests in Consolidated Funds			(270,186)	(50,778)
Net Cash Provided by (Used in) Financing Activities			(185,920)	27,415
Change in cash and cash equivalents			99,141	150,119
Total cash beginning of period			264,208	125,356	$ 125,356
Cash and cash equivalents	193,066	117,824	193,066	117,824	130,052
Cash collateral pledged	16,092	16,995	16,092	16,995	17,888
Segregated cash	154,191	140,656	154,191	140,656	116,268
Total cash at end of period	$ 363,349	$ 275,475	363,349	275,475	$ 264,208
Supplemental information
Cash paid during the year for interest			62,371	24,528
Cash paid during the year for taxes			4,862	1,944
Purchase of treasury stock, at cost, through net settlement			8,544	11,433
Preferred stock dividends declared			5,094	5,094
Net assets (liabilities) acquired upon acquisition (net of cash)			0	(50,017)
Transfer of investment from consolidated funds, securities owned, fair value to securities owned, fair value			8,820	0
Net decrease in redeemable non-controlling interests in Consolidated Funds due to deconsolidation of consolidated fund			32,559	0
Separately recognized conversion option reclassification from a derivative liability to equity			28,974	0
Common stock issuance upon close of acquisition			0	47,607
Conversion of redeemable non-controlling interest to loan receivable			$ 0	$ 1,299