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Allowance for Loan Losses - Average Recorded Investment and Interest Income Recognized 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, Impaired [Line Items] |  |  |  |
Impaired financing receivable, Average Recorded Investment | $ 10,738 | $ 10,409 | $ 10,696 | $ 10,371
Impaired financing receivable, Interest Income Recognized | 165 | 164 | 330 | 325
Private Education Loans - Traditional [Member] |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |
Impaired financing receivable, Average Recorded Investment | 9,320 | 8,943 | 9,271 | 8,900
Impaired financing receivable, Interest Income Recognized | 138 | 135 | 276 | 267
Private Education Loans - Non-Traditional [Member] |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |
Impaired financing receivable, Average Recorded Investment | 1,418 | 1,466 | 1,425 | 1,471
Impaired financing receivable, Interest Income Recognized | $ 27 | $ 29 | $ 54 | $ 58

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- Definition Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5258-111524 + Details
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