Borrowings - Financing VIEs (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Securities Financing Transaction [Line Items]
Short-term borrowings	$ 3,802	$ 2,490
Long-term borrowings	70,825	74,488
Total	74,627	76,978
Total assets	78,158	80,605
Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	3,799	2,490
Long-term borrowings	70,937	74,231
Total	74,736	76,721
Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	3
Long-term borrowings	(112)	257
Total	(109)	257
Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	2,596	2,188
Long-term borrowings	64,755	67,107
Total	67,351	69,295
Loans	69,003	70,649
Cash	2,471	2,649
Other assets	1,500	1,522
Total assets	72,974	74,820
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Long-term borrowings	49,622	51,841
Total	49,622	51,841
Loans	50,024	52,066
Cash	1,860	2,073
Other assets	1,535	1,520
Total assets	53,419	55,659
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	515	543
Long-term borrowings	14,038	14,074
Total	14,553	14,617
Loans	15,467	15,506
Cash	518	505
Other assets	138	150
Total assets	16,123	16,161
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	619	282
Long-term borrowings	145	150
Total	764	432
Loans	755	436
Cash	14	8
Other assets	26	15
Total assets	795	459
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,462	1,363
Long-term borrowings	1,114	1,152
Total	2,576	2,515
Loans	2,757	2,641
Cash	79	63
Other assets	30	32
Total assets	2,866	2,736
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	2,596	2,188
Long-term borrowings	64,919	67,217
Total	67,515	69,405
Loans	69,003	70,649
Cash	2,471	2,649
Other assets	1,729	1,717
Total assets	73,203	75,015
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Long-term borrowings	(164)	(110)
Total	(164)	(110)
Other assets	(229)	(195)
Total assets	$ (229)	$ (195)