GUARANTEES AND COMMITMENTS - Credit Commitments and Lines of Credit (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Guarantor Obligations
Credit commitments	$ 1,044,925	$ 985,174
Unsettled reverse repurchase and securities borrowing agreements	54,100	35,000
Unsettled repurchase and securities lending agreements	43,000	19,100
Commercial and similar letters of credit
Guarantor Obligations
Credit commitments	5,088	5,000
One- to four-family residential mortgages
Guarantor Obligations
Credit commitments	2,908	2,674
Revolving open-end loans secured by one- to four-family residential properties
Guarantor Obligations
Credit commitments	11,603	12,323
Commercial real estate, construction and land development
Guarantor Obligations
Credit commitments	14,146	11,151
Credit card lines
Guarantor Obligations
Credit commitments	700,260	678,300
Commercial and other consumer loan commitments
Guarantor Obligations
Credit commitments	307,239	272,655
Other commitments and contingencies
Guarantor Obligations
Credit commitments	3,681	$ 3,071
U.S.
Guarantor Obligations
Credit commitments	832,885
U.S. | Commercial and similar letters of credit
Guarantor Obligations
Credit commitments	798
U.S. | One- to four-family residential mortgages
Guarantor Obligations
Credit commitments	1,199
U.S. | Revolving open-end loans secured by one- to four-family residential properties
Guarantor Obligations
Credit commitments	10,212
U.S. | Commercial real estate, construction and land development
Guarantor Obligations
Credit commitments	12,175
U.S. | Credit card lines
Guarantor Obligations
Credit commitments	605,614
U.S. | Commercial and other consumer loan commitments
Guarantor Obligations
Credit commitments	199,722
U.S. | Other commitments and contingencies
Guarantor Obligations
Credit commitments	3,165
Outside of U.S.
Guarantor Obligations
Credit commitments	212,040
Outside of U.S. | Commercial and similar letters of credit
Guarantor Obligations
Credit commitments	4,290
Outside of U.S. | One- to four-family residential mortgages
Guarantor Obligations
Credit commitments	1,709
Outside of U.S. | Revolving open-end loans secured by one- to four-family residential properties
Guarantor Obligations
Credit commitments	1,391
Outside of U.S. | Commercial real estate, construction and land development
Guarantor Obligations
Credit commitments	1,971
Outside of U.S. | Credit card lines
Guarantor Obligations
Credit commitments	94,646
Outside of U.S. | Commercial and other consumer loan commitments
Guarantor Obligations
Credit commitments	107,517
Outside of U.S. | Other commitments and contingencies
Guarantor Obligations
Credit commitments	$ 516