Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
FFELP Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance for loans acquired	$ 0	$ 0	$ 0	$ 0
Loans receivable, discount	34,000,000	40,000,000	34,000,000	40,000,000
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance for loans acquired	0	0	0	0
Loans receivable, discount	$ 298,000,000	$ 362,000,000	$ 298,000,000	$ 362,000,000

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.

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of addition to allowance reserve for credit loss arising from purchase of loan and lease portfolios.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Disclosure of the unamortized amount of the discount on the note or receivable which is deducted from the face amount of the receivable or loan. The discount or premium is the difference between the present value and the face amount.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
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- Details

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