Consolidated Statements of Cash Flows - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Cash flows from operating activities:
Net income (loss)	$ 6,583,000	$ 19,598,000	$ 26,596,000	$ 75,538,000
Adjustments to reconcile net income (loss) to net cash provided by / (used in) operating activities:
Depreciation and amortization	5,082,000	3,139,000	14,990,000	9,558,000
Amortization of debt issuance costs			804,000	990,000
Amortization of debt discount			3,511,000	4,177,000
Noncash lease expense			(2,607,000)	0
Gain / (loss) on extinguishment of debt			0	(652,000)
Share-based compensation			27,224,000	27,636,000
Change in deferred taxes			8,420,000	14,640,000
Deferred rent obligations			0	(2,532,000)
Net loss (gain) on disposal of fixed assets			367,000	(146,000)
Goodwill Impairment	0	0	4,100,000	0	$ 0
Purchases of securities owned, at fair value			(1,405,478,000)	(3,137,168,000)
Proceeds from sales of securities owned, at fair value			1,282,527,000	3,329,707,000
Proceeds from sales of securities sold, not yet purchased, at fair value			902,271,000	2,132,935,000
Payments to cover securities sold, not yet purchased, at fair value			(939,072,000)	(2,233,427,000)
Proceeds from other investments			19,693,000	7,313,000
Net (gains) losses on securities, derivatives and other investments			(57,808,000)	(61,927,000)
Consolidated Funds
Purchases of securities owned, at fair value			(2,016,225,000)	(457,293,000)
Proceeds from sales of securities owned, at fair value			1,880,160,000	491,427,000
Purchases of other investments			(2,798,000)	(1,414,000)
Proceeds from other investments			23,865,000	230,832,000
Net realized and unrealized (gains) losses on investments and other transactions			(51,925,000)	(58,523,000)
(Increase) decrease in operating assets:
Securities owned, at fair value, held at broker-dealer			(250,579,000)	(124,740,000)
Receivable on derivative contracts, at fair value			(14,629,000)	36,949,000
Securities borrowed			(820,241,000)	(270,140,000)
Deposits with clearing organizations, brokers and banks			(2,027,000)	(4,650,000)
Receivable from brokers, dealers and clearing organizations			68,479,000	(136,214,000)
Receivable from customers, net of allowance			(8,268,000)	16,651,000
Fees receivable, net of allowance			17,707,000	(6,211,000)
Due from related parties			6,020,000	2,765,000
Other assets			1,038,000	16,573,000
Consolidated Funds
Cash and cash equivalents			34,809,000	(130,000)
Receivable on derivative contracts, at fair value			531,000	609,000
Receivable from brokers			(9,579,000)	(5,373,000)
Other assets			10,000	(153,000)
Increase (decrease) in operating liabilities:
Securities sold, not yet purchased, at fair value, held at broker dealer			191,640,000	146,853,000
Payable for derivative contracts, at fair value			16,672,000	16,225,000
Securities loaned			924,795,000	91,798,000
Payable to brokers, dealers and clearing organizations			42,281,000	(41,827,000)
Payable to customers			(88,000)	118,009,000
Commission management payable			(6,146,000)	19,782,000
Compensation payable			(92,903,000)	33,395,000
Fees payable			11,060,000	14,211,000
Due to related parties			(4,402,000)	1,000
Accounts payable, accrued expenses and other liabilities			(2,679,000)	1,527,000
Consolidated Funds
Payable to brokers			(8,355,000)	1,185,000
Payable for derivative contracts, at fair value			6,115,000	(4,941,000)
Due To Related Parties, Consolidated Funds			615,000	0
Accounts payable, accrued expenses and other liabilities			(184,000)	205,000
Net Cash Provided by (Used in) Operating Activities			(156,655,000)	295,626,000
Cash flows from investing activities:
Purchases of other investments			(15,899,000)	(21,355,000)
Purchase of business			(48,581,000)	0
Proceeds from sales of other investments			18,103,000	15,730,000
Proceeds from loans held for investment			0	1,410,000
Purchase of fixed assets			(7,862,000)	(6,710,000)
Net Cash Provided by (Used in) Investing Activities			(54,239,000)	(10,925,000)
Cash flows from financing activities:
Proceeds from (Payments for) in Securities Sold under Agreements to Repurchase			23,772,000	0
Repayments on convertible debt			(20,860,000)	(13,500,000)
Deferred debt issuance cost			(1,480,000)	(3,985,000)
Borrowings on notes and other debt			84,640,000	102,382,000
Repayments on notes and other debt			(7,834,000)	(6,561,000)
Purchase of treasury stock			(10,153,000)	(16,763,000)
Contingent liability payment			(1,235,000)	(797,000)
Capital contributions by redeemable non-controlling interests in operating entities			11,110,000	941,000
Capital withdrawals to redeemable non-controlling interests in operating entities			(1,713,000)	(1,685,000)
Consolidated Funds
Capital contributions by redeemable non-controlling interests in Consolidated Funds			213,723,000	24,594,000
Capital withdrawals to redeemable non-controlling interests in Consolidated Funds			(89,032,000)	(270,186,000)
Net Cash Provided by (Used in) Financing Activities			177,166,000	(185,560,000)
Change in cash and cash equivalents			(33,728,000)	99,141,000
Total cash beginning of period			442,113,000	264,208,000	264,208,000
Cash and cash equivalents	260,994,000	193,066,000	260,994,000	193,066,000	259,148,000
Cash collateral pledged	18,609,000	16,092,000	18,609,000	16,092,000	6,318,000
Segregated cash	128,782,000	154,191,000	128,782,000	154,191,000	176,647,000
Total cash at end of period	408,385,000	363,349,000	408,385,000	363,349,000	442,113,000
Supplemental information
Cash paid during the year for interest			98,363,000	62,371,000
Cash paid during the year for taxes			3,815,000	4,862,000
Purchase of treasury stock, at cost, through net settlement			22,420,000	8,544,000
Preferred stock dividends	1,698,000	1,698,000	5,094,000	5,094,000
Net assets (liabilities) acquired upon acquisition (net of cash)			90,727,000	0
Operating right of use assets	96,664,000		96,664,000		0
Operating lease liabilities	$ 100,934,000		100,934,000		$ 0
Transfer of investment from consolidated funds, securities owned, fair value to securities owned, fair value			97,655,000	8,820,000
Net decrease in redeemable non-controlling interests in Consolidated Funds due to deconsolidation of consolidated fund				32,559,000
Separately recognized conversion option reclassification from a derivative liability to equity		$ 1,000	(596,000)	28,974,000
Common stock issuance upon close of acquisition			$ 14,446,000	$ 0