v3.26.1
Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Earning assets:    
Held for sale loans $ 2,166 $ 13,822
Trading investments 55,938 49,250
Accrued interest receivable, difference 99,155 99,829
Derivative instruments 0 0
Total earning assets, difference 2,782,028 2,979,661
Interest-bearing liabilities:    
Short-term borrowings, difference 0 8,613
Long-term borrowings, difference 58,521 (14,415)
Derivative instruments 2 8
Total interest-bearing liabilities, difference 118,956 35,883
Excess of net asset fair value over carrying value 2,900,984 3,015,544
Money-market and savings accounts    
Interest-bearing liabilities:    
Deposits, difference 22,559 (5,449)
Certificates of deposit    
Interest-bearing liabilities:    
Deposits, difference 37,876 47,134
Fair Value    
Earning assets:    
Held for sale loans 174,632 947,078
Cash and cash equivalents 4,588,129 4,241,265
Trading investments 55,938 49,250
Available-for-sale investments 1,716,712 1,758,070
Accrued interest receivable 1,701,466 1,662,640
Derivative instruments 1 0
Total earning assets 30,449,078 31,856,437
Interest-bearing liabilities:    
Short-term borrowings 0 489,802
Long-term borrowings 5,785,595 5,376,909
Accrued interest payable 97,204 97,524
Derivative instruments 3 8
Total interest-bearing liabilities 25,716,657 26,982,525
Fair Value | Money-market and savings accounts    
Interest-bearing liabilities:    
Deposits 10,609,001 11,187,471
Fair Value | Certificates of deposit    
Interest-bearing liabilities:    
Deposits 9,224,854 9,830,811
Carrying Value    
Earning assets:    
Held for sale loans 172,466 933,256
Cash and cash equivalents 4,588,129 4,241,265
Trading investments 55,938 49,250
Available-for-sale investments 1,716,712 1,758,070
Accrued interest receivable 1,602,311 1,562,811
Derivative instruments 1 0
Total earning assets 27,667,050 28,876,776
Interest-bearing liabilities:    
Short-term borrowings 0 498,415
Long-term borrowings 5,844,116 5,362,494
Accrued interest payable 97,204 97,524
Derivative instruments 3 8
Total interest-bearing liabilities 25,835,613 27,018,408
Carrying Value | Money-market and savings accounts    
Interest-bearing liabilities:    
Deposits 10,631,560 11,182,022
Carrying Value | Certificates of deposit    
Interest-bearing liabilities:    
Deposits 9,262,730 9,877,945
Private Education Loans    
Earning assets:    
Loans held for investment, net, difference 2,680,707 2,866,010
Private Education Loans | Fair Value    
Earning assets:    
Loans held for investment, net 22,212,200 23,198,134
Private Education Loans | Carrying Value    
Earning assets:    
Loans held for investment, net $ 19,531,493 $ 20,332,124