Borrowings - Financing VIEs (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	$ 4,226	$ 5,870
Long-term borrowings, Carrying Value	53,402	61,026
Total	57,628	66,896
Ending Balance of education loans	54,827	62,250
Total assets	61,375	70,795
Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	4,235	5,880
Long-term borrowings, Carrying Value	53,680	61,516
Total	57,915	67,396
Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	(9)	(10)
Long-term borrowings, Carrying Value	(278)	(490)
Total	(287)	(500)
Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	3,634	4,458
Long-term borrowings, Carrying Value	48,169	55,598
Total	51,803	60,056
Ending Balance of education loans	53,591	60,672
Cash	1,937	3,233
Other assets, Net	1,744	1,356
Total assets	57,272	65,261
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	59	76
Long-term borrowings, Carrying Value	35,626	42,675
Total	35,685	42,751
Ending Balance of education loans	35,935	42,148
Cash	1,441	2,705
Other assets, Net	1,673	1,544
Total assets	39,049	46,397
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	435	725
Long-term borrowings, Carrying Value	11,754	12,744
Total	12,189	13,469
Ending Balance of education loans	13,396	14,168
Cash	350	367
Other assets, Net	119	105
Total assets	13,865	14,640
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	1,854	923
Long-term borrowings, Carrying Value	89	386
Total	1,943	1,309
Ending Balance of education loans	1,897	1,317
Cash	77	39
Other assets, Net	92	44
Total assets	2,066	1,400
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan ABCP Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	1,286	2,734
Long-term borrowings, Carrying Value	821	0
Total	2,107	2,734
Ending Balance of education loans	2,363	3,039
Cash	69	122
Other assets, Net	50	(81)
Total assets	2,482	3,080
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	3,634	4,458
Long-term borrowings, Carrying Value	48,290	55,805
Total	51,924	60,263
Ending Balance of education loans	53,591	60,672
Cash	1,937	3,233
Other assets, Net	1,934	1,612
Total assets	57,462	65,517
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings, Carrying Value	0	0
Long-term borrowings, Carrying Value	(121)	(207)
Total	(121)	(207)
Ending Balance of education loans	0	0
Cash	0	0
Other assets, Net	(190)	(256)
Total assets	$ (190)	$ (256)