| Interest Income and Interest Expense Disclosure | 
 Net interest income/expense includes: 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
 | 
 Year ended 
March 31, 2025 
 | 
 | 
 Year ended 
March 31, 2024  
 | 
 | 
Year ended March 31, 2023  | 
 
| Interest income: | 
 | 
 | 
 | 
 | 
 | 
 
| Interest income on trading securities | 
$ | 
378,350  | 
 | 
 | 
$ | 
426,428  | 
 | 
 | 
$ | 
178,288  | 
 | 
 
| Interest income on margin loans to customers | 
212,360  | 
 | 
 | 
175,571  | 
 | 
 | 
34,558  | 
 | 
 
| Interest income on loans to customers | 
207,802  | 
 | 
 | 
176,539  | 
 | 
 | 
43,486  | 
 | 
 
Interest income on available-for-sale securities  | 
40,297  | 
 | 
 | 
32,821  | 
 | 
 | 
27,003  | 
 | 
 
| Interest income on reverse repurchase agreements and amounts due from banks | 
24,644  | 
 | 
 | 
16,865  | 
 | 
 | 
9,836  | 
 | 
 
Interest income on held-to-maturity securities  | 
1,000  | 
 | 
 | 
—  | 
 | 
 | 
—  | 
 | 
 
Other interest income  | 
—  | 
 | 
 | 
—  | 
 | 
 | 
1,524  | 
 | 
 
| Total interest income | 
$ | 
864,453 
 | 
 | 
 | 
$ | 
828,224 
 | 
 | 
 | 
$ | 
294,695 
 | 
 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Interest expense: | 
 | 
 | 
 | 
 | 
 | 
 
| Interest expense on securities repurchase agreement obligations | 
$ | 
340,863  | 
 | 
 | 
$ | 
402,665  | 
 | 
 | 
$ | 
158,595  | 
 | 
 
| Interest expense on customer accounts and deposits | 
111,541  | 
 | 
 | 
70,778  | 
 | 
 | 
40,335  | 
 | 
 
| Interest expense on margin lending payable | 
45,748  | 
 | 
 | 
16,990  | 
 | 
 | 
6,635  | 
 | 
 
| Interest expense on debt securities issued | 
36,130  | 
 | 
 | 
10,356  | 
 | 
 | 
3,085  | 
 | 
 
| Interest expense on loans received | 
1,568  | 
 | 
 | 
124  | 
 | 
 | 
290  | 
 | 
 
| Other interest expense | 
45  | 
 | 
 | 
198  | 
 | 
 | 
7  | 
 | 
 
| Total interest expense | 
$ | 
535,895 
 | 
 | 
 | 
$ | 
501,111 
 | 
 | 
 | 
$ | 
208,947 
 | 
 | 
 
 | 
 | 
 | 
 | 
 | 
 | 
 
| Net interest income | 
$ | 
328,558 
 | 
 | 
 | 
$ | 
327,113 
 | 
 | 
 | 
$ | 
85,748 
 | 
 | 
 
  
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