v3.26.1
Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Jun. 30, 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Total earning assets, Carrying Value $ 44,504 $ 45,862  
Short-term borrowings, Carrying Value 4,214 5,073  
Long-term borrowings, Carrying Value 40,123 40,633  
Total 44,337 45,706  
Total earning assets, Fair Value 43,843 45,417  
Short-term borrowings, Fair Value 4,224 5,084  
Long-term borrowings, Fair Value 39,374 40,120  
Total interest-bearing liabilities, Fair Value 43,598 45,204  
FFELP Loans [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loans, Carrying Value 26,575 28,141  
Loans receivable, Fair Value 26,499 28,096  
Interest Rate Swaps [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Derivative financial instruments, Carrying Value 26 39  
Derivative financial instruments, Fair Value 26 39  
Cross-Currency Interest Rate Swaps [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Derivative financial instruments, Carrying Value (91) (76)  
Derivative financial instruments, Fair Value (91) (76)  
Private Education Loans [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loans, Carrying Value 15,146   $ 15,530
Private Education Loans [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loans, Carrying Value 15,674 15,451  
Loans receivable, Fair Value 15,089 15,051  
Difference [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Cash and investments 0 0  
Total earning assets, Fair Value (661) (445)  
Short-term borrowings, Fair Value (10) (11)  
Long-term borrowings, Fair Value 749 513  
Total interest-bearing liabilities, Fair Value 739 502  
Excess of net asset fair value over carrying value 78 57  
Difference [Member] | FFELP Loans [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loans receivable, Fair Value (76) (45)  
Difference [Member] | Interest Rate Swaps [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Derivative financial instruments, Fair Value 0 0  
Difference [Member] | Cross-Currency Interest Rate Swaps [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Derivative financial instruments, Fair Value 0 0  
Difference [Member] | Private Education Loans [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loans receivable, Fair Value (585) (400)  
Carrying Value [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Cash and investments 2,255 2,270  
Fair Value [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Cash and investments $ 2,255 $ 2,270