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Allowance for Loan Losses - Average Recorded Investment and Interest Income Recognized for TDR (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Impaired [Line Items]
Impaired financing receivable, Average Recorded Investment	$ 10,769	$ 10,444	$ 10,721	$ 10,396
Impaired financing receivable, Interest Income Recognized	167	163	497	488
Private Education Loans - Traditional [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivable, Average Recorded Investment	9,367	8,988	9,303	8,930
Impaired financing receivable, Interest Income Recognized	140	134	416	402
Private Education Loans - Non-Traditional [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivable, Average Recorded Investment	1,402	1,456	1,418	1,466
Impaired financing receivable, Interest Income Recognized	$ 27	$ 29	$ 81	$ 86

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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

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

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- Definition

Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ 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)(3)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

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