FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Items Measured at Fair Value on a Nonrecurring Basis
Non-marketable equity securities measured using the measurement alternative	$ 7,332	$ 7,523
Nonrecurring | Level 2
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	478	3,249
Other real estate owned	4	6
Loans	679	93
Non-marketable equity securities measured using the measurement alternative	312	249
Total assets	1,473	3,597
Nonrecurring | Level 3
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	2,897	1,330
Other real estate owned	13	14
Loans	336	251
Non-marketable equity securities measured using the measurement alternative	3	0
Total assets	3,249	1,595
Fair value | Nonrecurring
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	3,375	4,579
Other real estate owned	17	20
Loans	1,015	344
Non-marketable equity securities measured using the measurement alternative	315	249
Total assets	$ 4,722	$ 5,192