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Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total earning assets, Carrying Value	$ 47,360	$ 48,814
Short-term borrowings, Carrying Value	4,752	5,134
Long-term borrowings, Carrying Value	42,345	43,184
Total	47,097	48,318
Total earning assets, Fair Value	46,689	48,379
Short-term borrowings, Fair Value	4,764	5,144
Long-term borrowings, Fair Value	41,672	42,361
Total interest-bearing liabilities, Fair Value	46,436	47,505
FFELP Loans [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, Carrying Value	29,618	30,852
Loans receivable, Fair Value	29,261	30,766
Interest Rate Swaps [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial instruments, Carrying Value	43	25
Derivative financial instruments, Fair Value	43	25
Cross-Currency Interest Rate Swaps [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial instruments, Carrying Value	(75)	(244)
Derivative financial instruments, Fair Value	(75)	(244)
Private Education Loans [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, Carrying Value	15,530	15,716	$ 16,238
Loans receivable, Fair Value	15,216	15,367
Difference [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and investments	0	0
Total earning assets, Fair Value	(671)	(435)
Short-term borrowings, Fair Value	(12)	(10)
Long-term borrowings, Fair Value	673	823
Total interest-bearing liabilities, Fair Value	661	813
Excess of net asset fair value over carrying value	(10)	378
Difference [Member] | FFELP Loans [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable, Fair Value	(357)	(86)
Difference [Member] | Interest Rate Swaps [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial instruments, Fair Value	0	0
Difference [Member] | Cross-Currency Interest Rate Swaps [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial instruments, Fair Value	0	0
Difference [Member] | Private Education Loans [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable, Fair Value	(314)	(349)
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and investments	2,212	2,246
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and investments	$ 2,212	$ 2,246

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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.

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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-10

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

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