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Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 23,497	$ 24,333
School Type/FICO Scores [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 23,497	$ 24,333
School Type/FICO Scores [Member] | Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators, Percent	100.00%	100.00%
School Type/FICO Scores [Member] | Private Education Loans [Member] | Traditional Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 21,601	$ 22,367
Total Private Education Loans Credit Quality Indicators, Percent	92.00%	92.00%
School Type/FICO Scores [Member] | Private Education Loans [Member] | Non Traditional Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 1,896	$ 1,966
Total Private Education Loans Credit Quality Indicators, Percent	8.00%	8.00%
Cosigners [Member] | Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators, Percent	100.00%	100.00%
Cosigners [Member] | Private Education Loans [Member] | Cosigner Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 15,024	$ 15,610
Total Private Education Loans Credit Quality Indicators, Percent	64.00%	64.00%
Cosigners [Member] | Private Education Loans [Member] | Non Cosigner Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 8,473	$ 8,723
Total Private Education Loans Credit Quality Indicators, Percent	36.00%	36.00%
Seasoning [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 23,497	$ 24,333
Seasoning [Member] | Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators, Percent	100.00%	100.00%
Seasoning [Member] | Private Education Loans [Member] | 1-12 Payments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 1,276	$ 1,340
Total Private Education Loans Credit Quality Indicators, Percent	5.00%	5.00%
Seasoning [Member] | Private Education Loans [Member] | 13-24 Payments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 1,130	$ 1,271
Total Private Education Loans Credit Quality Indicators, Percent	5.00%	5.00%
Seasoning [Member] | Private Education Loans [Member] | 25-36 Payments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 1,684	$ 1,908
Total Private Education Loans Credit Quality Indicators, Percent	7.00%	8.00%
Seasoning [Member] | Private Education Loans [Member] | 37-48 Payments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 2,440	$ 2,723
Total Private Education Loans Credit Quality Indicators, Percent	10.00%	11.00%
Seasoning [Member] | Private Education Loans [Member] | More than 48 Payments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 15,630	$ 15,698
Total Private Education Loans Credit Quality Indicators, Percent	67.00%	65.00%
Seasoning [Member] | Private Education Loans [Member] | Not Yet in Repayment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans Credit Quality Indicators	$ 1,337	$ 1,393
Total Private Education Loans Credit Quality Indicators, Percent	6.00%	6.00%

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522

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