Borrowings - Company's Borrowings (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Short-term Debt	$ 7,078	$ 8,483
Long-term debt	79,137	81,715
Total Before Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term Debt	7,067	8,479
Long-term debt	78,681	81,756
Total Before Hedge Accounting Adjustments [Member] | FFELP Loan Asset-backed Securitization Repurchase Facilities [Member]
Debt Instrument [Line Items]
Short-term Debt	0	72
Long-term debt	133	231
Total Before Hedge Accounting Adjustments [Member] | Private Education Loan Asset-backed Securitization Repurchase Facilities [Member]
Debt Instrument [Line Items]
Short-term Debt	1,300	2,100
Long-term debt	260	194
Total Before Hedge Accounting Adjustments [Member] | Short Term Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt principal amount	1,800	1,100
Total Before Hedge Accounting Adjustments [Member] | Long Term Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt principal amount	$ 7,800	$ 8,500