DERIVATIVES - Effect of Derivative Instruments on Net Income and Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pre-tax net gains (losses) recognized in OCI	$ 91		$ 143
Amount of pretax net gains (losses) recognized in OCI before adoption of 2017-12		$ (17)		$ (87)
Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pretax net gains (losses) from OCI into income statement	(20)		(40)
Amount of pretax net gains (losses) from OCI into income/expense before adoption of 2017-12		(13)		(19)
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of pretax net gains (losses) from OCI into income statement	$ 1		$ 1
Amount of pretax net gains (losses) from OCI into income/expense before adoption of 2017-12		$ 4		$ 8