DERIVATIVES - Effect of Derivative Instruments on Net Income and Stockholders' Equity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pre-tax net gains (losses) recognized in OCI	$ 138
Amount of pre-tax net gains (losses) recognized in OCI, before adoption of 2017-12		$ (44)	$ (23)
Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pre-tax net losses reclassified	(68)
Amount of pre-tax net losses reclassified, before adoption of 2017-12		(55)	25
Interest expense - long-term borrowed funds
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pre-tax net losses reclassified	$ 11
Amount of pre-tax net losses reclassified, before adoption of 2017-12		$ 12	$ 0