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Borrowings - Company's Borrowings (Parenthetical) (Detail) - USD ($) $ in Millions | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | ---
Debt Instrument [Line Items] |  |
Long-term debt | $ 116,540 | $ 124,833
Total Before Hedge Accounting Adjustments [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 116,291 | 124,916
Private Education Loan Securitizations Repurchase Facility [Member] | Total Before Hedge Accounting Adjustments [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | $ 1,000 | $ 546

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