DERIVATIVES - Effect of Derivative Instruments on Net Income and Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pre-tax net (losses) gains recognized in OCI	$ (5)		$ 138
Amount of pretax net gains (losses) recognized in OCI before adoption of 2017-12		$ (35)		$ (122)
Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pretax net gains (losses) from OCI into income statement	(22)		(62)
Amount of pretax net gains (losses) from OCI into income/expense before adoption of 2017-12		(17)		(36)
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pretax net gains (losses) from OCI into income statement	$ 8		$ 9
Amount of pretax net gains (losses) from OCI into income/expense before adoption of 2017-12		$ 3		$ 11