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Allowance for Loan Losses - Recorded Investment, Unpaid Principal Balance and Related Allowance for TDR Loans (Detail) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Financing Receivable, Impaired [Line Items] |  |
TDR Loans, Recorded Investment | $ 10,684 | $ 10,759
TDR loans, Unpaid Principal Balance | 10,727 | 10,805
TDR Loans, Related Allowance | 1,176 | 1,190
Private Education Loans - Traditional [Member] |  |
Financing Receivable, Impaired [Line Items] |  |
TDR Loans, Recorded Investment | 9,336 | 9,386
TDR loans, Unpaid Principal Balance | 9,376 | 9,429
TDR Loans, Related Allowance | 994 | 1,003
Private Education Loans - Non-Traditional [Member] |  |
Financing Receivable, Impaired [Line Items] |  |
TDR Loans, Recorded Investment | 1,348 | 1,373
TDR loans, Unpaid Principal Balance | 1,351 | 1,376
TDR Loans, Related Allowance | $ 182 | $ 187

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- Definition Amount of allowance for credit losses related to recorded investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Recorded Investment -URI http://asc.fasb.org/extlink&oid=65880944 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524 + Details
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- Definition Amount of investment in impaired financing receivables for which there is 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 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953550-111524 + Details
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- Definition Unpaid principal balance of impaired financing receivables for which there is 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 (a)(4) -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524 + Details
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