Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues:
Commissions	$ 367,016	$ 352,027	$ 1,192,004	$ 1,190,522
Principal transactions	73,997	65,182	254,757	277,946
Fees from related parties	3,470	8,814	11,500	20,897
Data, software and post-trade	22,238	21,523	65,826	61,060
Interest and dividend income	3,042	2,418	17,535	13,115
Other revenues	3,984	5,078	12,151	13,795
Total revenues	473,747	455,042	1,553,773	1,577,335
Expenses:
Compensation and employee benefits	257,604	244,648	836,533	873,691
Equity-based compensation and allocations of net income to limited partnership units and FPUs	78,490	33,007	170,275	103,030
Total compensation and employee benefits	336,094	277,655	1,006,808	976,721
Occupancy and equipment	46,049	45,924	141,598	145,074
Fees to related parties	5,674	7,728	15,574	18,590
Professional and consulting fees	16,836	15,755	53,071	55,839
Communications	29,305	30,097	89,891	91,165
Selling and promotion	9,586	5,942	25,692	31,338
Commissions and floor brokerage	15,908	12,933	48,145	45,730
Interest expense	16,735	19,665	53,268	54,796
Other expenses	24,614	28,367	64,423	67,445
Total expenses	500,801	444,066	1,498,470	1,486,698
Other income (losses), net:
Gains (losses) on divestitures and sale of investments	92	(9)	60	(9)
Gains (losses) on equity method investments	1,816	1,527	4,605	3,669
Other income (loss)	4,513	4,779	11,783	(107)
Total other income (losses), net	6,421	6,297	16,448	3,553
Income (loss) from operations before income taxes	(20,633)	17,273	71,751	94,190
Provision (benefit) for income taxes	(6,692)	8,558	7,056	28,032
Consolidated net income (loss)	(13,941)	8,715	64,695	66,158
Less: Net income (loss) attributable to noncontrolling interest in subsidiaries	(2,539)	(135)	17,141	17,067
Net income (loss) available to common stockholders	(11,402)	8,850	47,554	49,091
Basic earnings (loss) per share
Net income (loss) available to common stockholders	$ (11,402)	$ 8,850	$ 47,554	$ 49,091
Basic earnings (loss) per share (in dollars per share)	$ (0.03)	$ 0.02	$ 0.12	$ 0.14
Basic weighted-average shares of common stock outstanding (in shares)	387,121	363,244	382,161	360,629
Fully diluted earnings (loss) per share
Net income (loss) for fully diluted shares	$ (11,402)	$ 8,850	$ 56,033	$ 71,609
Fully diluted earnings (loss) per share (in shares)	$ (0.03)	$ 0.02	$ 0.12	$ 0.13
Fully diluted weighted-average shares of common stock outstanding (in shares)	387,121	364,602	452,083	544,475