Borrowings - Financing VIEs (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Securities Financing Transaction [Line Items]
Short-term Debt	$ 6,785	$ 5,422
Long-term debt	86,776	93,519
Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term Debt	4,904	4,341
Long-term debt	77,373	82,738
Restricted cash	2,640	3,928
FFELP Loan Securitizations Repurchase Facilities [Member] | Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term Debt	72
Long-term debt	214	244
Restricted cash	9	9
Private Education Loan Securitizations Repurchase Facilities [Member] | Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term Debt	1,900	300
Long-term debt	921	2,000
Restricted cash	$ 65	$ 115