FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Items Measured at Fair Value on a Nonrecurring Basis
Non-marketable equity securities measured using the measurement alternative	$ 7,574	$ 7,212
Nonrecurring | Level 2
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	2,391	3,261
Other real estate owned	37	62
Loans	172	139
Non-marketable equity securities measured using the measurement alternative	108	192
Total assets	2,708	3,654
Nonrecurring | Level 3
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	1,243	1,794
Other real estate owned	13	16
Loans	310	251
Non-marketable equity securities measured using the measurement alternative	17	69
Total assets	1,583	2,130
Fair value | Nonrecurring
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	3,634	5,055
Other real estate owned	50	78
Loans	482	390
Non-marketable equity securities measured using the measurement alternative	125	261
Total assets	$ 4,291	$ 5,784