Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues:
Commissions	$ 389,768	$ 382,640	$ 824,988	$ 838,495
Principal transactions	81,997	99,453	180,760	212,764
Fees from related parties	4,245	6,562	8,030	12,083
Data, software and post-trade	21,602	20,139	43,588	39,537
Interest and dividend income	11,455	6,536	14,493	10,697
Other revenues	3,383	3,758	8,167	8,679
Total revenues	512,450	519,088	1,080,026	1,122,255
Expenses:
Compensation and employee benefits	270,427	283,616	578,589	628,544
Equity-based compensation and allocations of net income to limited partnership units and FPUs	58,290	27,819	91,785	70,023
Total compensation and employee benefits	328,717	311,435	670,374	698,567
Occupancy and equipment	46,900	47,247	95,033	98,321
Fees to related parties	4,452	5,194	9,743	10,629
Professional and consulting fees	17,820	19,805	33,960	39,761
Communications	30,774	30,524	60,578	61,045
Selling and promotion	8,616	6,634	16,104	25,333
Commissions and floor brokerage	14,308	13,520	32,237	32,797
Interest expense	18,680	17,625	36,533	35,131
Other expenses	23,688	21,480	39,777	39,011
Total expenses	493,955	473,464	994,339	1,040,595
Other income (losses), net:
Gains (losses) on divestitures and sale of investments	(32)	0	(32)	0
Gains (losses) on equity method investments	1,323	1,119	2,789	2,142
Other income (loss)	1,864	1,129	7,270	(4,886)
Total other income (losses), net	3,155	2,248	10,027	(2,744)
Income (loss) from operations before income taxes	21,650	47,872	95,714	78,916
Provision (benefit) for income taxes	(1,191)	8,599	13,748	19,474
Consolidated net income (loss)	22,841	39,273	81,966	59,442
Less: Net income (loss) attributable to noncontrolling interest in subsidiaries	4,672	11,354	20,706	17,849
Net income (loss) available to common stockholders	18,169	27,919	61,260	41,593
Basic earnings (loss) per share
Net income (loss) available to common stockholders	$ 18,169	$ 27,919	$ 61,260	$ 41,593
Basic earnings (loss) per share (in dollars per share)	$ 0.05	$ 0.08	$ 0.16	$ 0.12
Basic weighted-average shares of common stock outstanding (in shares)	384,902	360,614	379,639	359,308
Fully diluted earnings (loss) per share
Net income (loss) for fully diluted shares	$ 26,023	$ 40,173	$ 88,271	$ 59,498
Fully diluted earnings (loss) per share (in shares)	$ 0.05	$ 0.07	$ 0.16	$ 0.11
Fully diluted weighted-average shares of common stock outstanding (in shares)	563,923	546,123	560,210	542,390