DERIVATIVE INSTRUMENTS - Offsetting of Financial Liabilities and Derivative Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Offsetting Liabilities [Line Items]
Gross amount of recognized derivative liabilities	$ 257	$ 285
Gross amount of assets offset in balance sheet	0	0
Derivative liability, fair value, amount not offset against collateral	257	285
Financial instruments, amount not offset	(118)	(152)
Derivative, collateral, right to reclaim securities	(65)	(37)
Derivative, collateral, right to reclaim cash	(45)	(68)
Derivative liability, fair value, amount offset against collateral, subject to master netting agreement	29	28
Derivative liability, not subject to master netting arrangement	108	102
Derivative liability, fair value, amount offset against collateral, not subject to master netting agreement	108	102
Derivative liability, fair value, gross liability including not subject to master netting arrangement	365	387
Net amount of derivative liabilities presented in balance sheet	365	387
Derivative liability, fair value, amount offset against collateral	137	130
Gross Amounts of Recognized Financial Instruments, Offsetting Liabilities	2,334	1,439
Gross Amounts Offset in Statement of Financial Position, Offsetting Liabilities	0	0
Net Amounts of Financial Instruments Presented in Balance Sheet, Offsetting Liabilities	2,334	1,439
Carrying value of financial instruments, liabilities not offset in balance sheet	(2,077)	(1,181)
Securities Collateral, Liabilities Not Offset in Balance Sheet	(65)	(37)
Cash Collateral, Liabilities Not Offset in Balance Sheet	(45)	(68)
Financial instruments, amount of liabilities offset against collateral	147	153
Over the Counter - Bilateral
Offsetting Liabilities [Line Items]
Gross amount of recognized derivative liabilities	257	285
Gross amount of assets offset in balance sheet	0	0
Derivative liability, fair value, amount not offset against collateral	257	285
Financial instruments, amount not offset	(118)	(152)
Derivative, collateral, right to reclaim securities	(65)	(37)
Derivative, collateral, right to reclaim cash	(45)	(68)
Derivative liability, fair value, amount offset against collateral, subject to master netting agreement	29	28
Derivative liability, not subject to master netting arrangement	108	102
Derivative liability, fair value, amount offset against collateral, not subject to master netting agreement	108	102
Securities Lending and Similar Arrangements
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments, Offsetting Liabilities	1,969	1,052
Gross Amounts Offset in Statement of Financial Position, Offsetting Liabilities	0	0
Net Amounts of Financial Instruments Presented in Balance Sheet, Offsetting Liabilities	1,969	1,052
Carrying value of financial instruments, liabilities not offset in balance sheet	(1,959)	(1,029)
Securities Collateral, Liabilities Not Offset in Balance Sheet	0	0
Cash Collateral, Liabilities Not Offset in Balance Sheet	0	0
Financial instruments, amount of liabilities offset against collateral	$ 10	$ 23