FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Items Measured at Fair Value on a Nonrecurring Basis
Non-marketable equity securities measured using the measurement alternative	$ 8,349	$ 7,523
Nonrecurring | Level 2
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	781	3,249
Other real estate owned	8	6
Loans	553	93
Non-marketable equity securities measured using the measurement alternative	308	249
Total assets	1,650	3,597
Nonrecurring | Level 3
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	4,170	1,330
Other real estate owned	7	14
Loans	206	251
Non-marketable equity securities measured using the measurement alternative	0	0
Total assets	4,383	1,595
Fair value | Nonrecurring
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	4,951	4,579
Other real estate owned	15	20
Loans	759	344
Non-marketable equity securities measured using the measurement alternative	308	249
Total assets	$ 6,033	$ 5,192