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Allowance for Loan Losses - Loans Modified Accounts for TDR (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Financing Receivable, Modifications [Line Items] |  |  |  |
Modified Loans | $ 254 | $ 371 | $ 934 | $ 1,218
Charge-Offs | 86 | 107 | 282 | 357
Payment-Default | 67 | 98 | 206 | 313
Private Education Loans - Traditional [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Modified Loans | 233 | 339 | 861 | 1,107
Charge-Offs | 68 | 81 | 221 | 273
Payment-Default | 56 | 83 | 174 | 266
Private Education Loans - Non-Traditional [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Modified Loans | 21 | 32 | 73 | 111
Charge-Offs | 18 | 26 | 61 | 84
Payment-Default | $ 11 | $ 15 | $ 32 | $ 47

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- Definition Pre-modification amount of investment in financing receivables modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953676-111524 + Details
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- Definition Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953682-111524 + Details
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- Definition Reflects the amount of the write-down related to a troubled debt restructuring. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554 + Details
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