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Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Net income (loss) | $ 230 | $ 237 | $ 535 | $ 711
Education Loan Portfolio [Member] |  |  |  |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Loans unamortized premium (discount) | 451 |  | 451 |
Total loans gross including receivable for partially charged-off loans | 115,000 |  | 115,000 |
Education Loan Portfolio [Member] | Immaterial Error Correction Adjustments [Member] | Scenario, Plan [Member] |  |  |  |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Loans unamortized premium (discount) | $ (2) |  | (2) |
Net income (loss) |  |  | $ (1) |
FFELP Stafford Loans [Member] |  |  |  |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Constant prepayment rates | 5.00% |  | 5.00% |
FFELP Stafford Loans [Member] | Previous Interest Method Policy [Member] |  |  |  |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Constant prepayment rates | 5.00% |  | 5.00% |
FFELP Consolidation Loans [Member] |  |  |  |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Constant prepayment rates | 3.00% |  | 3.00% |
FFELP Consolidation Loans [Member] | Previous Interest Method Policy [Member] |  |  |  |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Constant prepayment rates | 3.00% |  | 3.00% |
Private Education Loans [Member] |  |  |  |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Total loans gross including receivable for partially charged-off loans | $ 25,864 | $ 29,377 | $ 25,864 | $ 29,377
Constant prepayment rates | 5.00% |  | 5.00% |
Private Education Loans [Member] | Previous Interest Method Policy [Member] |  |  |  |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |  |  |  |
Constant prepayment rates | 6.00% |  | 6.00% |

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