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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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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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

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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

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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

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