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Allowance for Loan Losses - Recorded Investment, Unpaid Principal Balance and Related Allowance for TDR Loans (Detail) - USD ($) $ in Millions | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | ---
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
TDR Loans, Recorded Investment | $ 10,785 | $ 10,575
TDR loans, Unpaid Principal Balance | 10,832 | 10,642
TDR Loans, Related Allowance | 1,153 | 1,209
Private Education Loans - Traditional [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
TDR Loans, Recorded Investment | 9,391 | 9,134
TDR loans, Unpaid Principal Balance | 9,434 | 9,200
TDR Loans, Related Allowance | 959 | 995
Private Education Loans - Non-Traditional [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
TDR Loans, Recorded Investment | 1,394 | 1,441
TDR loans, Unpaid Principal Balance | 1,398 | 1,442
TDR Loans, Related Allowance | $ 194 | $ 214

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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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