v3.26.3
Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Earning assets:    
Trading investments $ 41,308 $ 49,250
Accrued interest receivable, difference 53,274 99,829
Derivative instruments 1 0
Total earning assets 2,888,418 1,807,320
Total earning assets, difference 2,177,073 2,979,661
Interest-bearing liabilities:    
Short-term borrowings, difference 0 8,613
Long-term borrowings, difference 88,723 (14,415)
Derivative instruments 0 8
Total interest-bearing liabilities, difference 276,581 35,883
Excess of net asset fair value over carrying value 2,453,654 3,015,544
Money-market and savings accounts    
Interest-bearing liabilities:    
Deposits, difference 55,395 (5,449)
Certificates of deposit    
Interest-bearing liabilities:    
Deposits, difference 132,463 47,134
Fair Value    
Earning assets:    
Held for sale loans 1,455,605 947,078
Cash and cash equivalents 2,165,175 4,241,265
Trading investments 41,308 49,250
Available-for-sale investments 2,847,109 1,758,070
Accrued interest receivable 1,733,869 1,662,640
Derivative instruments 1 0
Total earning assets 30,979,536 31,856,437
Interest-bearing liabilities:    
Short-term borrowings 898,729 489,802
Long-term borrowings 5,447,200 5,376,909
Accrued interest payable 85,358 97,524
Derivative instruments 0 8
Total interest-bearing liabilities 27,003,149 26,982,525
Fair Value | Money-market and savings accounts    
Interest-bearing liabilities:    
Deposits 10,290,813 11,187,471
Fair Value | Certificates of deposit    
Interest-bearing liabilities:    
Deposits 10,281,049 9,830,811
Carrying Value    
Earning assets:    
Loans held for investment, net, difference 13,500 13,822
Held for sale loans 1,442,105 933,256
Cash and cash equivalents 2,165,175 4,241,265
Trading investments 41,308 49,250
Available-for-sale investments 2,847,109 1,758,070
Accrued interest receivable 1,680,595 1,562,811
Derivative instruments 1 0
Total earning assets 28,802,463 28,876,776
Interest-bearing liabilities:    
Short-term borrowings 898,729 498,415
Long-term borrowings 5,535,923 5,362,494
Accrued interest payable 85,358 97,524
Derivative instruments 0 8
Total interest-bearing liabilities 27,279,730 27,018,408
Carrying Value | Money-market and savings accounts    
Interest-bearing liabilities:    
Deposits 10,346,208 11,182,022
Carrying Value | Certificates of deposit    
Interest-bearing liabilities:    
Deposits 10,413,512 9,877,945
Private Education Loans    
Earning assets:    
Loans held for investment, net, difference 2,110,299 2,866,010
Private Education Loans | Fair Value    
Earning assets:    
Loans held for investment, net 22,736,469 23,198,134
Private Education Loans | Carrying Value    
Earning assets:    
Loans held for investment, net $ 20,626,170 $ 20,332,124