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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

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+ 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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