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Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total earning assets, Carrying Value | $ 94,168 | $ 99,986
Short-term borrowings, Carrying Value | 6,785 | 5,422
Long-term borrowings, Carrying Value | 86,776 | 93,519
Total | 93,561 | 98,941
Total earning assets, Fair Value | 95,338 | 100,520
Short-term borrowings, Fair Value | 6,812 | 5,418
Long-term borrowings, Fair Value | 86,412 | 92,173
Total interest-bearing liabilities, Fair Value | 93,224 | 97,591
FFELP Loans [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans, Carrying Value | 67,956 | 72,253
Loans receivable, Fair Value | 68,378 | 72,074
Floor Income Contracts [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Derivative financial instruments, Carrying Value | (99) | (53)
Derivative financial instruments, Fair Value | (99) | (53)
Interest Rate Swaps [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Derivative financial instruments, Carrying Value | 219 | 94
Derivative financial instruments, Fair Value | 219 | 94
Cross-Currency Interest Rate Swaps [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Derivative financial instruments, Carrying Value | (603) | (659)
Derivative financial instruments, Fair Value | (603) | (659)
Other [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Derivative financial instruments, Carrying Value | (2) | (4)
Derivative financial instruments, Fair Value | (2) | (4)
Private Education Loans [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans, Carrying Value | 21,564 | 22,245
Loans receivable, Fair Value | 22,312 | 22,958
Difference [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total earning assets, Fair Value | 1,170 | 534
Short-term borrowings, Fair Value | (27) | 4
Long-term borrowings, Fair Value | 364 | 1,346
Total interest-bearing liabilities, Fair Value | 337 | 1,350
Excess of net asset fair value over carrying value | 1,507 | 1,884
Difference [Member] | FFELP Loans [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable, Fair Value | 422 | (179)
Difference [Member] | Private Education Loans [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable, Fair Value | 748 | 713
Carrying Value [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and investments | 4,648 | 5,488
Fair Value [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and investments | $ 4,648 | $ 5,488

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- Definition Carrying values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments. + References No definition available. + Details
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- Definition Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities. + References No definition available. + Details
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- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: 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(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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