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Allowance for Loan Losses - Average Recorded Investment and Interest Income Recognized for TDR (Detail) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2017 | Mar. 31, 2016
Financing Receivable, Impaired [Line Items] |  |
Impaired financing receivable, Average Recorded Investment | $ 10,728 | $ 10,655
Impaired financing receivable, Interest Income Recognized | 172 | 165
Private Education Loans - Traditional [Member] |  |
Financing Receivable, Impaired [Line Items] |  |
Impaired financing receivable, Average Recorded Investment | 9,367 | 9,221
Impaired financing receivable, Interest Income Recognized | 145 | 138
Private Education Loans - Non-Traditional [Member] |  |
Financing Receivable, Impaired [Line Items] |  |
Impaired financing receivable, Average Recorded Investment | 1,361 | 1,434
Impaired financing receivable, Interest Income Recognized | $ 27 | $ 27

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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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