REVENUE RECOGNITION = Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Total revenue	$ 387,629	$ 327,784	$ 757,319	$ 654,861
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	335,761	287,287	656,111	577,047
Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	51,868	40,497	101,208	77,814
Transaction and clearing fees
Disaggregation of Revenue [Line Items]
Total revenue	331,884	286,139	647,294	575,443
Access fees
Disaggregation of Revenue [Line Items]
Total revenue	34,941	26,106	68,298	50,189
Market data fees
Disaggregation of Revenue [Line Items]
Total revenue	12,206	10,253	25,161	19,895
Other revenue
Disaggregation of Revenue [Line Items]
Total revenue	8,598	5,286	16,566	9,334
Corporate & Other
Disaggregation of Revenue [Line Items]
Total revenue	250	253	541	539
Corporate & Other | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Corporate & Other | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	250	253	541	539
Corporate & Other | Transaction and clearing fees
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Corporate & Other | Access fees
Disaggregation of Revenue [Line Items]
Total revenue	(57)	(57)	(114)	(114)
Corporate & Other | Market data fees
Disaggregation of Revenue [Line Items]
Total revenue	(7)	(7)	(15)	(15)
Corporate & Other | Other revenue
Disaggregation of Revenue [Line Items]
Total revenue	314	317	670	668
Options | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	316,494	261,759	621,946	522,853
Options | Operating Segments | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	278,609	232,790	548,363	467,389
Options | Operating Segments | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	37,885	28,969	73,583	55,464
Options | Operating Segments | Transaction and clearing fees
Disaggregation of Revenue [Line Items]
Total revenue	276,641	232,412	543,459	466,924
Options | Operating Segments | Access fees
Disaggregation of Revenue [Line Items]
Total revenue	30,756	22,208	59,874	42,592
Options | Operating Segments | Market data fees
Disaggregation of Revenue [Line Items]
Total revenue	8,996	6,878	18,478	13,076
Options | Operating Segments | Other revenue
Disaggregation of Revenue [Line Items]
Total revenue	101	261	135	261
Equities | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	40,579	40,364	74,544	80,438
Equities | Operating Segments | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	34,957	34,340	62,898	68,646
Equities | Operating Segments | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	5,622	6,024	11,646	11,792
Equities | Operating Segments | Transaction and clearing fees
Disaggregation of Revenue [Line Items]
Total revenue	34,957	34,339	62,845	68,646
Equities | Operating Segments | Access fees
Disaggregation of Revenue [Line Items]
Total revenue	3,797	3,674	7,664	7,154
Equities | Operating Segments | Market data fees
Disaggregation of Revenue [Line Items]
Total revenue	1,825	2,351	3,982	4,638
Equities | Operating Segments | Other revenue
Disaggregation of Revenue [Line Items]
Total revenue	0	0	53	0
Futures | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	24,562	23,117	48,900	47,914
Futures | Operating Segments | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	20,300	19,514	40,999	40,249
Futures | Operating Segments | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	4,262	3,603	7,901	7,665
Futures | Operating Segments | Transaction and clearing fees
Disaggregation of Revenue [Line Items]
Total revenue	20,221	19,311	40,864	39,760
Futures | Operating Segments | Access fees
Disaggregation of Revenue [Line Items]
Total revenue	406	239	796	476
Futures | Operating Segments | Market data fees
Disaggregation of Revenue [Line Items]
Total revenue	1,312	951	2,556	2,036
Futures | Operating Segments | Other revenue
Disaggregation of Revenue [Line Items]
Total revenue	2,623	2,616	4,684	5,642
International | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	5,744	2,291	11,388	3,117
International | Operating Segments | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	1,895	643	3,851	763
International | Operating Segments | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	3,849	1,648	7,537	2,354
International | Operating Segments | Transaction and clearing fees
Disaggregation of Revenue [Line Items]
Total revenue	65	77	126	113
International | Operating Segments | Access fees
Disaggregation of Revenue [Line Items]
Total revenue	39	42	78	81
International | Operating Segments | Market data fees
Disaggregation of Revenue [Line Items]
Total revenue	80	80	160	160
International | Operating Segments | Other revenue
Disaggregation of Revenue [Line Items]
Total revenue	$ 5,560	$ 2,092	$ 11,024	$ 2,763