FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Items Measured at Fair Value on a Nonrecurring Basis
Non-marketable equity securities measured using the measurement alternative	$ 7,769	$ 7,523
Nonrecurring | Level 2
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	571	3,249
Other real estate owned	5	6
Loans	509	93
Non-marketable equity securities measured using the measurement alternative	193	249
Total assets	1,278	3,597
Nonrecurring | Level 3
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	3,237	1,330
Other real estate owned	8	14
Loans	330	251
Non-marketable equity securities measured using the measurement alternative	3	0
Total assets	3,578	1,595
Fair value | Nonrecurring
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	3,808	4,579
Other real estate owned	13	20
Loans	839	344
Non-marketable equity securities measured using the measurement alternative	196	249
Total assets	$ 4,856	$ 5,192