SEGMENT REPORTING - Schedule of Certain Financial Information for the Company's Reportable Operating Segments and Corporate and Other (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Revenues	$ 387,629	$ 327,784	$ 757,319	$ 654,861
Less: Cost of revenue	246,516	223,122	487,613	458,315
Revenues less cost of revenues	141,113	104,662	269,706	196,546
Operating expenses:
Compensation and benefits	40,966	40,210	85,356	77,981
Information technology and communication costs	10,202	8,851	19,685	16,399
Depreciation and amortization	8,778	6,938	16,866	13,108
Professional fees and outside services	11,448	10,095	22,855	19,352
Litigation settlement	30,000	0	30,000	0
Acquisition-related costs	0	2,247	0	2,901
General, administrative, and other expenses	11,922	9,030	21,179	17,221
Total operating expenses	113,316	77,371	195,941	146,962
Operating income	27,797	27,291	73,765	49,584
Non-operating (expense) income:
Change in fair value of puttable common stock	0	(1,688)	0	(1,891)
Change in fair value of puttable warrants issued with debt	0	(1,486)	0	(917)
Interest income	5,254	1,418	9,640	2,713
Interest expense and amortization of debt issuance costs	(44)	(4,902)	(89)	(9,332)
Loss on sale of intangible assets	0	(2,054)	0	(2,054)
Unrealized loss on derivative and digital assets	(337)	(4,605)	(2,878)	(47,018)
Gain on sale of business	23	0	50,570	0
Other, net	(3,911)	10,681	(2,179)	12,360
Income before income tax provision	28,782	24,655	128,829	3,445
Operating Segments | Options
Segment Reporting [Line Items]
Revenues	316,494	261,759	621,946	522,853
Less: Cost of revenue	192,100	168,994	386,285	348,864
Revenues less cost of revenues	124,394	92,765	235,661	173,989
Operating expenses:
Compensation and benefits	18,945	18,409	39,995	35,074
Information technology and communication costs	4,597	3,799	8,900	7,145
Depreciation and amortization	4,494	3,405	8,820	6,467
Professional fees and outside services	4,860	4,185	9,718	8,249
Litigation settlement	22,500		22,500
Acquisition-related costs		0		0
General, administrative, and other expenses	4,064	3,774	7,994	7,278
Total operating expenses	59,460	33,572	97,927	64,213
Operating income	64,934	59,193	137,734	109,776
Non-operating (expense) income:
Change in fair value of puttable common stock		0		0
Change in fair value of puttable warrants issued with debt		0		0
Interest income	114	336	224	766
Interest expense and amortization of debt issuance costs	0	0	0	0
Loss on sale of intangible assets		0		0
Unrealized loss on derivative and digital assets	0	0	0	0
Gain on sale of business	0		0
Other, net	(1)	0	(1)	(1)
Income before income tax provision	65,047	59,529	137,957	110,541
Operating Segments | Equities
Segment Reporting [Line Items]
Revenues	40,579	40,364	74,544	80,438
Less: Cost of revenue	35,037	36,001	62,338	72,416
Revenues less cost of revenues	5,542	4,363	12,206	8,022
Operating expenses:
Compensation and benefits	3,915	3,213	8,080	6,639
Information technology and communication costs	1,656	1,737	3,201	3,300
Depreciation and amortization	1,306	1,553	2,362	3,040
Professional fees and outside services	327	490	599	1,020
Litigation settlement	0		0
Acquisition-related costs		0		0
General, administrative, and other expenses	674	475	1,354	1,131
Total operating expenses	7,878	7,468	15,596	15,130
Operating income	(2,336)	(3,105)	(3,390)	(7,108)
Non-operating (expense) income:
Change in fair value of puttable common stock		0		0
Change in fair value of puttable warrants issued with debt		0		0
Interest income	0	0	0	0
Interest expense and amortization of debt issuance costs	0	0	0	0
Loss on sale of intangible assets		0		0
Unrealized loss on derivative and digital assets	0	0	0	0
Gain on sale of business	0		0
Other, net	0	0	0	0
Income before income tax provision	(2,336)	(3,105)	(3,390)	(7,108)
Operating Segments | Futures
Segment Reporting [Line Items]
Revenues	24,562	23,117	48,900	47,914
Less: Cost of revenue	19,468	18,127	39,176	37,035
Revenues less cost of revenues	5,094	4,990	9,724	10,879
Operating expenses:
Compensation and benefits	9,701	11,834	20,142	23,309
Information technology and communication costs	3,041	2,513	5,955	4,490
Depreciation and amortization	1,800	984	3,363	1,963
Professional fees and outside services	256	719	561	1,464
Litigation settlement	0		0
Acquisition-related costs		0		0
General, administrative, and other expenses	2,986	1,753	4,840	3,727
Total operating expenses	17,784	17,803	34,861	34,953
Operating income	(12,690)	(12,813)	(25,137)	(24,074)
Non-operating (expense) income:
Change in fair value of puttable common stock		0		0
Change in fair value of puttable warrants issued with debt		0		0
Interest income	151	196	265	392
Interest expense and amortization of debt issuance costs	0	(35)	0	(70)
Loss on sale of intangible assets		0		0
Unrealized loss on derivative and digital assets	0	0	0	0
Gain on sale of business	0		0
Other, net	(3,205)	629	(1,875)	2,244
Income before income tax provision	(15,744)	(12,023)	(26,747)	(21,508)
Operating Segments | International
Segment Reporting [Line Items]
Revenues	5,744	2,291	11,388	3,117
Less: Cost of revenue	0	0	0	0
Revenues less cost of revenues	5,744	2,291	11,388	3,117
Operating expenses:
Compensation and benefits	2,297	1,495	4,528	2,612
Information technology and communication costs	620	555	1,073	1,013
Depreciation and amortization	551	446	986	603
Professional fees and outside services	308	390	770	576
Litigation settlement	0		0
Acquisition-related costs		0		0
General, administrative, and other expenses	874	555	1,674	918
Total operating expenses	4,650	3,441	9,031	5,722
Operating income	1,094	(1,150)	2,357	(2,605)
Non-operating (expense) income:
Change in fair value of puttable common stock		0		0
Change in fair value of puttable warrants issued with debt		0		0
Interest income	186	20	345	20
Interest expense and amortization of debt issuance costs	0	0	0	0
Loss on sale of intangible assets		(2,054)		(2,054)
Unrealized loss on derivative and digital assets	(337)	(4,605)	(2,878)	(47,018)
Gain on sale of business	0		0
Other, net	0	(62)	0	(62)
Income before income tax provision	943	(7,851)	(176)	(51,719)
Corporate & Other
Segment Reporting [Line Items]
Revenues	250	253	541	539
Less: Cost of revenue	(89)	0	(186)	0
Revenues less cost of revenues	339	253	727	539
Operating expenses:
Compensation and benefits	6,108	5,259	12,611	10,347
Information technology and communication costs	288	247	556	451
Depreciation and amortization	627	550	1,335	1,035
Professional fees and outside services	5,697	4,311	11,207	8,043
Litigation settlement	7,500		7,500
Acquisition-related costs		2,247		2,901
General, administrative, and other expenses	3,324	2,473	5,317	4,167
Total operating expenses	23,544	15,087	38,526	26,944
Operating income	(23,205)	(14,834)	(37,799)	(26,405)
Non-operating (expense) income:
Change in fair value of puttable common stock		(1,688)		(1,891)
Change in fair value of puttable warrants issued with debt		(1,486)		(917)
Interest income	4,803	866	8,806	1,535
Interest expense and amortization of debt issuance costs	(44)	(4,867)	(89)	(9,262)
Loss on sale of intangible assets		0		0
Unrealized loss on derivative and digital assets	0	0	0	0
Gain on sale of business	23		50,570
Other, net	(705)	10,114	(303)	10,179
Income before income tax provision	$ (19,128)	$ (11,895)	$ 21,185	$ (26,761)