FEE AND COMMISSION INCOME AND EXPENSE - Fee and commission income and expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2023
Schedule of Fee and Commission income [Line Items]
Brokerage services	$ 430,136	$ 333,383	$ 286,732
Commission income from payment processing	28,711	41,659	6,385
Agency fee income	15,616	15,872	87
Bank services	13,336	25,180	17,964
Underwriting and market-making services	11,210	18,801	11,948
Other fee and commission income	6,017	5,438	4,099
Fee and commission income (including $4,725, $65,972 and $199,235 from related parties)	505,026	440,333	327,215
Agency fee expense	284,055	103,020	22,807
Brokerage services	19,846	16,587	25,582
Bank services	20,064	18,121	12,272
Exchange services	1,776	3,302	2,631
Central Depository services	707	446	364
Other commission expenses	19,626	12,875	2,004
Total fee and commission expense	346,074	154,351	65,660
Brokerage
Schedule of Fee and Commission income [Line Items]
Brokerage services	430,136	333,383	286,732
Commission income from payment processing	0	0	0
Agency fee income	0	0	0
Bank services	0	0	0
Underwriting and market-making services	11,210	18,801	11,948
Other fee and commission income	493	297	390
Fee and commission income (including $4,725, $65,972 and $199,235 from related parties)	441,839	352,481	299,070
Agency fee expense	0	0	0
Brokerage services	19,846	16,506	25,551
Bank services	4,806	2,538	3,175
Exchange services	1,546	3,133	2,413
Central Depository services	706	446	364
Other commission expenses	2,376	2,117	1,623
Total fee and commission expense	29,280	24,740	33,126
Banking
Schedule of Fee and Commission income [Line Items]
Brokerage services	0	0	0
Commission income from payment processing	0	0	0
Agency fee income	0	108	0
Bank services	13,336	25,180	17,964
Underwriting and market-making services	0	0	0
Other fee and commission income	1,353	948	244
Fee and commission income (including $4,725, $65,972 and $199,235 from related parties)	14,689	26,236	18,208
Agency fee expense	36	0	0
Brokerage services	0	2	0
Bank services	14,419	13,217	9,097
Exchange services	0	0	0
Central Depository services	0	0	0
Other commission expenses	107	0	0
Total fee and commission expense	14,562	13,219	9,097
Insurance
Schedule of Fee and Commission income [Line Items]
Brokerage services	0	0	0
Commission income from payment processing	0	0	0
Agency fee income	0	296	87
Bank services	0	0	0
Underwriting and market-making services	0	0	0
Other fee and commission income	0	0	41
Fee and commission income (including $4,725, $65,972 and $199,235 from related parties)	0	296	128
Agency fee expense	284,019	103,020	22,807
Brokerage services	0	3	17
Bank services	538	208	0
Exchange services	0	0	0
Central Depository services	1	0	0
Other commission expenses	22	115	0
Total fee and commission expense	284,580	103,346	22,824
Other
Schedule of Fee and Commission income [Line Items]
Brokerage services	0	0	0
Commission income from payment processing	28,711	41,659	6,385
Agency fee income	15,616	15,468	0
Bank services	0	0	0
Underwriting and market-making services	0	0	0
Other fee and commission income	4,171	4,193	3,424
Fee and commission income (including $4,725, $65,972 and $199,235 from related parties)	48,498	61,320	9,809
Agency fee expense	0	0	0
Brokerage services	0	76	14
Bank services	301	2,158	0
Exchange services	230	169	218
Central Depository services	0	0	0
Other commission expenses	17,121	10,643	381
Total fee and commission expense	$ 17,652	$ 13,046	$ 613