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Allowance for Loan Losses - Loans Modified Accounts for TDR (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015
Financing Receivable, Modifications [Line Items] |  |  |  |
Modified Loans | $ 311 | $ 375 | $ 680 | $ 847
Charge-Offs | 94 | 131 | 196 | 250
Payment-Default | 66 | 97 | 139 | 215
Private Education Loans - Traditional [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Modified Loans | 286 | 339 | 628 | 768
Charge-Offs | 74 | 101 | 154 | 192
Payment-Default | 56 | 83 | 118 | 183
Private Education Loans - Non-Traditional [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Modified Loans | 25 | 36 | 52 | 79
Charge-Offs | 20 | 30 | 42 | 58
Payment-Default | $ 10 | $ 14 | $ 21 | $ 32

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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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