DERIVATIVES - Collateral Amounts Not Offset in the Consolidated Balance Sheets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2019
Assets
Net Amount Presented in the Balance Sheets	$ 4,936	$ 6,470
Liabilities
Net Amount Presented in the Balance Sheets	9,226	12,383
Securities Sold under Agreements to Repurchase
Net Amount Presented in the Balance Sheets	573	0
Interest expense, securities sold under agreements to repurchase	2
Total derivatives
Assets
Net Amount Presented in the Balance Sheets	1,397	2,546
Collateral Held, Financial Instruments	0	367
Collateral (Received) Held, Cash	1,363	2,118
Net Amount	34	61
Liabilities
Net Amount Presented in the Balance Sheets	197	47
Collateral Held, Financial Instruments	0	0
Collateral Held, Cash	0	0
Net Amount	197	47
Other financial instruments
Assets
Net Amount Presented in the Balance Sheets	1,520	1,688
Collateral Held, Financial Instruments	0	0
Collateral (Received) Held, Cash	0	0
Net Amount	1,520	1,688
Other invested assets
Assets
Net Amount Presented in the Balance Sheets	2,917	4,234
Collateral Held, Financial Instruments	0	367
Collateral (Received) Held, Cash	1,363	2,118
Net Amount	1,554	1,749
Other financial liabilities
Liabilities
Net Amount Presented in the Balance Sheets	1,263	1,553
Collateral Held, Financial Instruments	0	0
Collateral Held, Cash	0	0
Net Amount	1,263	1,553
Other liabilities
Liabilities
Net Amount Presented in the Balance Sheets	1,460	1,600
Collateral Held, Financial Instruments	0	0
Collateral Held, Cash	0	0
Net Amount	1,460	$ 1,600
Securities sold under agreement to repurchase
Securities Sold under Agreements to Repurchase
Net Amount Presented in the Balance Sheets	571
Collateral Held, Financial Instruments	(588)
Collateral Held, Cash	0
Net Amount	$ (17)