| Interest Income and Interest Expense Disclosure | Net interest income/expense for the three months ended September 30, 2024, and September 30, 2023 included the following: 
 
|  |  |  |  |  |  |  |  |  |  |  |  |  
|  | Three months ended September 30, 2024 |  | Three months ended September 30, 2023 |  
| Interest income: |  |  |  |  
| Interest income on trading securities | 104,992 |  |  | $ | 114,039 |  |  
| Interest income on loans to customers | 49,393 |  |  | 42,868 |  |  
| Interest income on margin loans to customers | 40,917 |  |  | 42,573 |  |  
| Interest income on securities available-for-sale | 9,003 |  |  | 9,653 |  |  
| Interest income on reverse repurchase agreements and amounts due from banks | 6,019 |  |  | 3,930 |  |  
| Total interest income | $ | 210,324 |  |  | $ | 213,063 |  |  
|  |  |  |  |  
| Interest expense: |  |  |  |  
| Interest expense on securities repurchase agreement obligations | $ | 86,116 |  |  | $ | 117,009 |  |  
| Interest expense on customer accounts and deposits | 23,696 |  |  | 17,864 |  |  
| Interest expense on margin lending payable | 7,416 |  |  | 3,467 |  |  
| Interest expense on debt securities issued | 6,969 |  |  | 961 |  |  
| Interest expense on loans received | 468 |  |  | 26 |  |  
| Other interest expense | — |  |  | 54 |  |  
| Total interest expense | $ | 124,665 |  |  | $ | 139,381 |  |  
|  |  |  |  |  
| Net interest income | $ | 85,659 |  |  | $ | 73,682 |  | 
 Net interest income/expense for the six months ended September 30, 2024, and September 30, 2023 includes: 
|  |  |  |  |  |  |  |  |  |  |  |  |  
|  | Six months ended September 30, 2024 |  | Six months ended September 30, 2023 |  
| Interest income: |  |  |  |  
| Interest income on trading securities | $ | 212,120 |  |  | $ | 200,880 |  |  
| Interest income on loans to customers | 101,760 |  |  | 74,201 |  |  
| Interest income on margin loans to customers | 91,984 |  |  | 59,753 |  |  
| Interest income on securities available-for-sale | 17,403 |  |  | 17,997 |  |  
| Interest income on reverse repurchase agreements and amounts due from banks | 13,061 |  |  | 6,987 |  |  
| Other interest income | — |  |  | 2,594 |  |  
| Total interest income | $ | 436,328 |  |  | $ | 362,412 |  |  
|  |  |  |  |  
| Interest expense: |  |  |  |  
| Interest expense on securities repurchase agreement obligations | $ | 178,523 |  |  | $ | 192,464 |  |  
| Interest expense on customer accounts and deposits | 46,823 |  |  | 33,467 |  |  
| Interest expense on margin lending payable | 30,539 |  |  | 6,460 |  |  
| Interest expense on debt securities issued | 13,938 |  |  | 1,896 |  |  
| Interest expense on loans received | 512 |  |  | 53 |  |  
| Other interest expense | 48 |  |  | 87 |  |  
| Total interest expense | $ | 270,383 |  |  | $ | 234,427 |  |  
|  |  |  |  |  
| Net interest income | $ | 165,945 |  |  | $ | 127,985 |  |  |