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Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Loans, Carrying Value $ 105,122 $ 111,070
Total earning assets, Carrying Value 110,156 116,273
Short-term borrowings, Carrying Value 4,771 2,334
Long-term borrowings, Carrying Value 105,012 112,368
Total interest-bearing liabilities, Carrying Value 109,783 114,702
Total earning assets, Fair Value 111,726 115,020
Short-term borrowings, Fair Value 4,783 2,346
Long-term borrowings, Fair Value 104,921 109,826
Total interest-bearing liabilities, Fair Value 109,704 112,172
FFELP Loans [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Loans, Carrying Value 81,703 87,730
Loans receivable, Fair Value 82,271 86,626
Floor Income Contracts [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative financial instruments, Carrying Value (74) (184)
Derivative financial instruments, Fair Value (74) (184)
Interest Rate Swaps [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative financial instruments, Carrying Value 203 357
Derivative financial instruments, Fair Value 203 357
Cross-Currency Interest Rate Swaps [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative financial instruments, Carrying Value (366) (1,296)
Derivative financial instruments, Fair Value (366) (1,296)
Other [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative financial instruments, Carrying Value (18) (13)
Derivative financial instruments, Fair Value (18) (13)
Private Education Loans [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Loans, Carrying Value 23,419 23,340
Loans receivable, Fair Value 24,421 23,191
Difference [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Excess of net asset fair value over carrying value 1,649 1,277
Total earning assets, Fair Value 1,570 (1,253)
Short-term borrowings, Fair Value (12) (12)
Long-term borrowings, Fair Value 91 2,542
Total interest-bearing liabilities, Fair Value 79 2,530
Difference [Member] | FFELP Loans [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Loans receivable, Fair Value 568 (1,104)
Difference [Member] | Private Education Loans [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Loans receivable, Fair Value 1,002 (149)
Carrying Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash and investments 5,034 5,203
Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash and investments $ 5,034 $ 5,203