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Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Net income (loss)	$ 230	$ 237	$ 535	$ 711
Education Loan Portfolio [Member]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Loans unamortized premium (discount)	451		451
Total loans gross including receivable for partially charged-off loans	115,000		115,000
Education Loan Portfolio [Member] | Immaterial Error Correction Adjustments [Member] | Scenario, Plan [Member]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Loans unamortized premium (discount)	$ (2)		(2)
Net income (loss)			$ (1)
FFELP Stafford Loans [Member]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Constant prepayment rates	5.00%		5.00%
FFELP Stafford Loans [Member] | Previous Interest Method Policy [Member]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Constant prepayment rates	5.00%		5.00%
FFELP Consolidation Loans [Member]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Constant prepayment rates	3.00%		3.00%
FFELP Consolidation Loans [Member] | Previous Interest Method Policy [Member]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Constant prepayment rates	3.00%		3.00%
Private Education Loans [Member]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Total loans gross including receivable for partially charged-off loans	$ 25,864	$ 29,377	$ 25,864	$ 29,377
Constant prepayment rates	5.00%		5.00%
Private Education Loans [Member] | Previous Interest Method Policy [Member]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Constant prepayment rates	6.00%		6.00%

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 55
-Paragraph 4K
-URI http://asc.fasb.org/extlink&oid=65897068&loc=SL4591552-111686

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
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Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 310
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