| Interest Income and Interest Expense Disclosure | Net interest income/expense for the three months ended December 31, 2023, and December 31, 2022 includes: 
 
|  |  |  |  |  |  |  |  |  |  |  |  |  
|  | Three Months Ended December 31, 2023 |  | Three Months Ended December 31, 2022 |  
| Interest income: |  |  |  |  
| Interest income on trading securities | $ | 112,860 |  |  | $ | 44,760 |  |  
| Interest income on margin loans to customers | 51,553 |  |  | 12,379 |  |  
| Interest income on loans to customers | 49,529 |  |  | 12,327 |  |  
| Interest income on securities available-for-sale | 7,478 |  |  | 6,727 |  |  
| Interest income on reverse repurchase agreements and amounts due from banks | 5,025 |  |  | $ | 4,062 |  |  
| Total interest income | $ | 226,445 |  |  | $ | 80,255 |  |  
|  |  |  |  |  
| Interest expense: |  |  |  |  
| Interest expense on securities repurchase agreement obligations | $ | 110,778 |  |  | $ | 39,958 |  |  
| Interest expense on customer accounts and deposits | 15,653 |  |  | 11,149 |  |  
| Interest expense on margin lending payable | 3,211 |  |  | — |  |  
| Interest expense on debt securities issued | 1,491 |  |  | 842 |  |  
| Interest expense on loans received | 36 |  |  | 88 |  |  
| Other interest expense | 54 |  |  | — |  |  
| Total interest expense | $ | 131,223 |  |  | $ | 52,037 |  |  
|  |  |  |  |  
| Net interest income | $ | 95,222 |  |  | $ | 28,218 |  | Net interest income/expense for the nine months ended December 31, 2023, and December 31, 2022 includes: 
|  |  |  |  |  |  |  |  |  |  |  |  |  
|  | Nine Months Ended December 31, 2023 |  | Nine Months Ended December 31, 2022 |  
| Interest income: |  |  |  |  
| Interest income on trading securities | $ | 313,739 |  |  | $ | 116,922 |  |  
| Interest income on loans to customers | 123,730 |  |  | 24,158 |  |  
| Interest income on margin loans to customers | 111,306 |  |  | 27,259 |  |  
| Interest income on securities available-for-sale | 25,476 |  |  | 13,280 |  |  
| Interest income on reverse repurchase agreements and amounts due from banks | 12,012 |  |  | 6,198 |  |  
| Other interest income | 2,594 |  |  | — |  |  
| Total interest income | $ | 588,857 |  |  | $ | 187,817 |  |  
|  |  |  |  |  
| Interest expense: |  |  |  |  
| Interest expense on securities repurchase agreement obligations | $ | 303,242 |  |  | $ | 105,466 |  |  
| Interest expense on customer accounts and deposits | 49,120 |  |  | 24,780 |  |  
| Interest expense on margin lending payable | 9,671 |  |  | — |  |  
| Interest expense on debt securities issued | 3,387 |  |  | 2,457 |  |  
| Interest expense on loans received | 89 |  |  | 268 |  |  
| Other interest expense | 141 |  |  | — |  |  
| Total interest expense | $ | 365,650 |  |  | $ | 132,971 |  |  
|  |  |  |  |  
| Net interest income | $ | 223,207 |  |  | $ | 54,846 |  |  |