INVESTMENTS - Held-to-Maturity Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	$ 30,733	$ 30,318
Gross Unrealized Gains	8,105	6,470
Gross Unrealized Losses	16	66
Fair Value	38,822	36,722
Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	30,733	30,318
Gross Unrealized Gains	8,105	6,470
Gross Unrealized Losses	16	66
Fair Value	38,822	36,722
Japan government and agencies | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	22,581	21,712
Gross Unrealized Gains	6,653	5,326
Gross Unrealized Losses	0	0
Fair Value	29,234	27,038
Municipalities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	835	359
Fair Value	1,114	469
Municipalities | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	835	359
Gross Unrealized Gains	279	110
Gross Unrealized Losses	0	0
Fair Value	1,114	469
Mortgage- and asset-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	18	14
Fair Value	19	15
Mortgage- and asset-backed securities | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	18	14
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Fair Value	19	15
Public utilities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,573	2,727
Fair Value	2,967	2,973
Public utilities | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,573	2,727
Gross Unrealized Gains	394	254
Gross Unrealized Losses	0	8
Fair Value	2,967	2,973
Sovereign and supranational
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,187	1,551
Fair Value	1,384	1,840
Sovereign and supranational | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,187	1,551
Gross Unrealized Gains	197	289
Gross Unrealized Losses	0	0
Fair Value	1,384	1,840
Banks/financial institutions
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	930	1,445
Fair Value	1,030	1,583
Banks/financial institutions | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	930	1,445
Gross Unrealized Gains	108	158
Gross Unrealized Losses	8	20
Fair Value	1,030	1,583
Other corporate
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,609	2,510
Fair Value	3,074	2,804
Other corporate | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,609	2,510
Gross Unrealized Gains	473	332
Gross Unrealized Losses	8	38
Fair Value	$ 3,074	$ 2,804