Borrowings - Financing VIEs (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Securities Financing Transaction [Line Items]
Short-term borrowings	$ 6,785	$ 5,422
Long-term borrowings	86,776	93,519
Total	93,561	98,941
Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	6,762	5,425
Long-term borrowings	86,840	93,868
Total	93,602	99,293
Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	23	(3)
Long-term borrowings	(64)	(349)
Total	(41)	(352)
Assets and Liabilities of Consolidated Variable Interest Entities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,904	4,341
Long-term borrowings	77,373	82,738
Total	82,277	87,079
Loans	86,735	91,619
Cash	2,640	3,928
Other assets, Net	1,009	956
Total Carrying Amount of Assets Securing Debt Outstanding	90,384	96,503
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	62,405	66,318
Total	62,477	66,318
Loans	63,494	66,266
Cash	1,969	3,181
Other assets, Net	1,316	1,211
Total Carrying Amount of Assets Securing Debt Outstanding	66,779	70,658
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan Securitizations [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	1,867	300
Long-term borrowings	12,541	12,985
Total	14,408	13,285
Loans	16,433	16,336
Cash	541	536
Other assets, Net	173	198
Total Carrying Amount of Assets Securing Debt Outstanding	17,147	17,070
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | FFELP Loan - Other Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	2,270	2,927
Long-term borrowings	1,836	2,625
Total	4,106	5,552
Loans	4,206	5,656
Cash	66	132
Other assets, Net	124	162
Total Carrying Amount of Assets Securing Debt Outstanding	4,396	5,950
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Private Education Loan - Other Facilities [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	695	1,114
Long-term borrowings	1,053	1,266
Total	1,748	2,380
Loans	2,602	3,361
Cash	64	79
Other assets, Net	18	27
Total Carrying Amount of Assets Securing Debt Outstanding	2,684	3,467
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Total Before Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Short-term borrowings	4,904	4,341
Long-term borrowings	77,835	83,194
Total	82,739	87,535
Loans	86,735	91,619
Cash	2,640	3,928
Other assets, Net	1,631	1,598
Total Carrying Amount of Assets Securing Debt Outstanding	91,006	97,145
Assets and Liabilities of Consolidated Variable Interest Entities [Member] | Hedge Accounting Adjustments [Member]
Securities Financing Transaction [Line Items]
Long-term borrowings	(462)	(456)
Total	(462)	(456)
Other assets, Net	(622)	(642)
Total Carrying Amount of Assets Securing Debt Outstanding	$ (622)	$ (642)