DERIVATIVE INSTRUMENTS - Offsetting of Financial Assets and Derivative Assets (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Offsetting Assets [Line Items]
Gross amount of recognized derivative assets	$ 325	$ 234
Gross amount of liabilities offset in balance sheet	0	0
Net amount of derivative assets presented in balance sheet	325	234
Financial instruments, amount not offset	(141)	(152)
Derivative, collateral, obligation to return securities	(101)	(23)
Derivative, collateral, obligation to return cash	(69)	(58)
Derivative asset, fair value, amount offset against collateral, subject to master netting agreement	14	1
Derivative asset, not subject to master netting arrangement	201	183
Derivative asset, fair value, amount offset against collateral, not subject to master netting agreement	201	183
Derivative asset, fair value, gross asset including not subject to master netting arrangement	526	417
Net amount of derivative assets presented in balance sheet	526	417
Derivative asset, fair value, amount offset against collateral	215	184
Gross amounts of recognized financial instruments	2,062	1,446
Gross amounts offset in balance sheet	0	0
Net amounts of assets presented in balance sheet	2,062	1,446
Carrying value of financial instruments not offset in balance sheet	(141)	(152)
Securities collateral, not offset in balance sheet	(101)	(23)
Cash collateral, not offset in balance sheet	(1,605)	(1,087)
Financial instruments, amount of assets offset against collateral	215	184
Over the Counter - Bilateral
Offsetting Assets [Line Items]
Gross amount of recognized derivative assets	310	231
Gross amount of liabilities offset in balance sheet	0	0
Net amount of derivative assets presented in balance sheet	310	231
Financial instruments, amount not offset	(141)	(152)
Derivative, collateral, obligation to return securities	(101)	(23)
Derivative, collateral, obligation to return cash	(62)	(55)
Derivative asset, fair value, amount offset against collateral, subject to master netting agreement	6	1
Derivative asset, not subject to master netting arrangement	201	183
Derivative asset, fair value, amount offset against collateral, not subject to master netting agreement	201	183
Other the Counter - Cleared
Offsetting Assets [Line Items]
Gross amount of recognized derivative assets	15	3
Gross amount of liabilities offset in balance sheet	0	0
Net amount of derivative assets presented in balance sheet	15	3
Financial instruments, amount not offset	0	0
Derivative, collateral, obligation to return securities	0	0
Derivative, collateral, obligation to return cash	(7)	(3)
Derivative asset, fair value, amount offset against collateral, subject to master netting agreement	8	0
Securities Lending and Similar Arrangements
Offsetting Assets [Line Items]
Gross amounts of recognized financial instruments	1,536	1,029
Gross amounts offset in balance sheet	0	0
Net amounts of assets presented in balance sheet	1,536	1,029
Carrying value of financial instruments not offset in balance sheet	0	0
Securities collateral, not offset in balance sheet	0	0
Cash collateral, not offset in balance sheet	(1,536)	(1,029)
Financial instruments, amount of assets offset against collateral	$ 0	$ 0