FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Items Measured at Fair Value on a Nonrecurring Basis
Non-marketable equity securities measured using the measurement alternative	$ 7,344	$ 7,332
Nonrecurring | Level 2
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	212	478
Other real estate owned	0	4
Loans	0	679
Non-marketable equity securities measured using the measurement alternative	152	312
Total assets	364	1,473
Nonrecurring | Level 3
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	375	2,897
Other real estate owned	22	13
Loans	354	336
Non-marketable equity securities measured using the measurement alternative	335	3
Total assets	1,086	3,249
Fair value | Nonrecurring
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	587	3,375
Other real estate owned	22	17
Loans	354	1,015
Non-marketable equity securities measured using the measurement alternative	487	315
Total assets	$ 1,450	$ 4,722