FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Items Measured at Fair Value on a Nonrecurring Basis
Liabilities transferred from Level 1 to Level 2	$ 100	$ 300
Nonrecurring | Level 2
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	2,002	3,389
Other real estate owned	13	15
Loans	630	586
Total assets	2,645	3,990
Nonrecurring | Level 3
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	1,788	2,413
Other real estate owned	57	60
Loans	565	790
Total assets	2,410	3,263
Fair value | Nonrecurring
Items Measured at Fair Value on a Nonrecurring Basis
Loans held-for-sale	3,790	5,802
Other real estate owned	70	75
Loans	1,195	1,376
Total assets	$ 5,055	$ 7,253