INVESTMENTS - Securities Lending Transactions Accounted for as Secured Borrowings (Details) - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		$ 2,969	$ 1,876
Gross amount of recognized liabilities for securities lending		2,969	1,876
Japan government and agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		2,318	1,013
Public utilities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		35	35
Sovereign and supranational
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		1	2
Banks/financial institutions
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		68	48
Other corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		547	778
Maturity Overnight and Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	[1]	651	863
Maturity Overnight and Continuous | Japan government and agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	[1]	0	0
Maturity Overnight and Continuous | Public utilities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	[1]	35	35
Maturity Overnight and Continuous | Sovereign and supranational
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	[1]	1	2
Maturity Overnight and Continuous | Banks/financial institutions
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	[1]	68	48
Maturity Overnight and Continuous | Other corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	[1]	547	778
Maturity up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		2,318	1,013
Maturity up to 30 Days | Japan government and agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		2,318	1,013
Maturity up to 30 Days | Public utilities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		0	0
Maturity up to 30 Days | Sovereign and supranational
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		0	0
Maturity up to 30 Days | Banks/financial institutions
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		0	0
Maturity up to 30 Days | Other corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred		$ 0	$ 0

[1] The related loaned security, under the Company's U.S. securities lending program, can be returned to the Company at the transferee's discretion; therefore, they are classified as Overnight and Continuous.