FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Items Measured at Fair Value on a Nonrecurring Basis
Non-marketable equity securities measured using the measurement alternative	$ 7,212	$ 8,245
Nonrecurring | Level 2
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	3,261	2,066
Other real estate owned	62	10
Loans	139	216
Non-marketable equity securities measured using the measurement alternative	192
Total assets	3,654	2,292
Nonrecurring | Level 3
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	1,794	3,609
Other real estate owned	16	44
Loans	251	414
Non-marketable equity securities measured using the measurement alternative	69
Total assets	2,130	4,067
Fair value | Nonrecurring
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	5,055	5,675
Other real estate owned	78	54
Loans	390	630
Non-marketable equity securities measured using the measurement alternative	261
Total assets	$ 5,784	$ 6,359