Borrowings - Financing VIEs (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Securities Financing Transaction [Line Items]
Short-term borrowings	$ 7,078	$ 8,483
Long-term borrowings	79,137	81,715
Total	86,215	90,198
Total assets	89,664	94,903
Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	7,067	8,479
Long-term borrowings	78,681	81,756
Total	85,748	90,235
Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	11	4
Long-term borrowings	456	(41)
Total	467	(37)
Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,959	7,089
Long-term borrowings	70,641	72,856
Total	75,600	79,945
Loans	77,919	83,864
Cash	2,405	2,506
Other assets, Net	1,308	1,089
Total assets	81,632	87,459
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings		72
Long-term borrowings	55,354	59,735
Total	55,354	59,807
Loans	56,044	60,834
Cash	1,637	1,833
Other assets, Net	1,448	1,400
Total assets	59,129	64,067
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,297	2,120
Long-term borrowings	13,499	11,430
Total	14,796	13,550
Loans	15,814	15,412
Cash	638	509
Other assets, Net	196	152
Total assets	16,648	16,073
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	2,304	2,783
Long-term borrowings	843	617
Total	3,147	3,400
Loans	3,178	3,421
Cash	50	63
Other assets, Net	110	102
Total assets	3,338	3,586
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,358	2,114
Long-term borrowings	1,276	1,513
Total	2,634	3,627
Loans	2,883	4,197
Cash	80	101
Other assets, Net	29	28
Total assets	2,992	4,326
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,959	7,089
Long-term borrowings	70,972	73,295
Total	75,931	80,384
Loans	77,919	83,864
Cash	2,405	2,506
Other assets, Net	1,783	1,682
Total assets	82,107	88,052
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Long-term borrowings	(331)	(439)
Total	(331)	(439)
Other assets, Net	(475)	(593)
Total assets	$ (475)	$ (593)