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Borrowings - Company's Borrowings (Parenthetical) (Detail) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Debt Instrument [Line Items]    
Long-term debt $ 75,814 $ 77,332
Short-term Debt 4,068 6,613
Total Before Hedge Accounting Adjustments [Member]    
Debt Instrument [Line Items]    
Long-term debt 75,340 76,781
Short-term Debt 4,038 6,609
Total Before Hedge Accounting Adjustments [Member] | FFELP Loan Asset-backed Securitization Repurchase Facilities [Member]    
Debt Instrument [Line Items]    
Long-term debt 155 157
Total Before Hedge Accounting Adjustments [Member] | Private Education Loan Asset-backed Securitization Repurchase Facilities [Member]    
Debt Instrument [Line Items]    
Long-term debt 0 260
Short-term Debt 832 960
Total Before Hedge Accounting Adjustments [Member] | Short Term Senior Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Debt principal amount 1,700 678
Total Before Hedge Accounting Adjustments [Member] | Long Term Senior Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Debt principal amount $ 6,500 $ 7,800