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Borrowings - Company's Borrowings (Parenthetical) (Detail) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Debt Instrument [Line Items] |  |
Short-term Debt | $ 6,785 | $ 5,422
Long-term debt | 86,776 | 93,519
Total Before Hedge Accounting Adjustments [Member] |  |
Debt Instrument [Line Items] |  |
Short-term Debt | 6,762 | 5,425
Long-term debt | 86,840 | 93,868
Total Before Hedge Accounting Adjustments [Member] | FFELP Loan Securitizations Asset-backed Repurchase Facilities [Member] |  |
Debt Instrument [Line Items] |  |
Short-term Debt | 72 | 0
Long-term debt | 214 | 244
Total Before Hedge Accounting Adjustments [Member] | Private Education Loan Securitizations Repurchase Facilities [Member] |  |
Debt Instrument [Line Items] |  |
Short-term Debt | 1,900 | 300
Long-term debt | 921 | 2,000
Total Before Hedge Accounting Adjustments [Member] | Short Term Senior Unsecured Debt [Member] |  |
Debt Instrument [Line Items] |  |
Debt principal amount | 1,500 | 817
Total Before Hedge Accounting Adjustments [Member] | Long Term Senior Unsecured Debt [Member] |  |
Debt Instrument [Line Items] |  |
Debt principal amount | $ 9,100 | $ 10,800

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