Borrowings - Company's Borrowings (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Short-term Debt	$ 6,785	$ 5,422
Long-term debt	86,776	93,519
Total Before Hedge Accounting Adjustments [Member]
Debt Instrument [Line Items]
Short-term Debt	6,762	5,425
Long-term debt	86,840	93,868
Total Before Hedge Accounting Adjustments [Member] | FFELP Loan Securitizations Asset-backed Repurchase Facilities [Member]
Debt Instrument [Line Items]
Short-term Debt	72	0
Long-term debt	214	244
Total Before Hedge Accounting Adjustments [Member] | Private Education Loan Securitizations Repurchase Facilities [Member]
Debt Instrument [Line Items]
Short-term Debt	1,900	300
Long-term debt	921	2,000
Total Before Hedge Accounting Adjustments [Member] | Short Term Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt principal amount	1,500	817
Total Before Hedge Accounting Adjustments [Member] | Long Term Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt principal amount	$ 9,100	$ 10,800