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Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016
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Private Education Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 23,497 | $ 24,333
School Type/FICO Scores [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 23,497 | $ 24,333
School Type/FICO Scores [Member] | Private Education Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators, Percent | 100.00% | 100.00%
School Type/FICO Scores [Member] | Private Education Loans [Member] | Traditional Loan [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 21,601 | $ 22,367
Total Private Education Loans Credit Quality Indicators, Percent | 92.00% | 92.00%
School Type/FICO Scores [Member] | Private Education Loans [Member] | Non Traditional Loan [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 1,896 | $ 1,966
Total Private Education Loans Credit Quality Indicators, Percent | 8.00% | 8.00%
Cosigners [Member] | Private Education Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators, Percent | 100.00% | 100.00%
Cosigners [Member] | Private Education Loans [Member] | Cosigner Loan [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 15,024 | $ 15,610
Total Private Education Loans Credit Quality Indicators, Percent | 64.00% | 64.00%
Cosigners [Member] | Private Education Loans [Member] | Non Cosigner Loan [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 8,473 | $ 8,723
Total Private Education Loans Credit Quality Indicators, Percent | 36.00% | 36.00%
Seasoning [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 23,497 | $ 24,333
Seasoning [Member] | Private Education Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators, Percent | 100.00% | 100.00%
Seasoning [Member] | Private Education Loans [Member] | 1-12 Payments [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 1,276 | $ 1,340
Total Private Education Loans Credit Quality Indicators, Percent | 5.00% | 5.00%
Seasoning [Member] | Private Education Loans [Member] | 13-24 Payments [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 1,130 | $ 1,271
Total Private Education Loans Credit Quality Indicators, Percent | 5.00% | 5.00%
Seasoning [Member] | Private Education Loans [Member] | 25-36 Payments [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 1,684 | $ 1,908
Total Private Education Loans Credit Quality Indicators, Percent | 7.00% | 8.00%
Seasoning [Member] | Private Education Loans [Member] | 37-48 Payments [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 2,440 | $ 2,723
Total Private Education Loans Credit Quality Indicators, Percent | 10.00% | 11.00%
Seasoning [Member] | Private Education Loans [Member] | More than 48 Payments [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 15,630 | $ 15,698
Total Private Education Loans Credit Quality Indicators, Percent | 67.00% | 65.00%
Seasoning [Member] | Private Education Loans [Member] | Not Yet in Repayment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total Private Education Loans Credit Quality Indicators | $ 1,337 | $ 1,393
Total Private Education Loans Credit Quality Indicators, Percent | 6.00% | 6.00%

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