INVESTMENTS - Held-to-Maturity Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	$ 30,163	$ 30,318
Gross Unrealized Gains	7,298	6,470
Gross Unrealized Losses	15	66
Fair Value	37,446	36,722
Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	30,163	30,318
Gross Unrealized Gains	7,298	6,470
Gross Unrealized Losses	15	66
Fair Value	37,446	36,722
Japan government and agencies | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	21,961	21,712
Gross Unrealized Gains	6,018	5,326
Gross Unrealized Losses	0	0
Fair Value	27,979	27,038
Municipalities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	814	359
Fair Value	1,061	469
Municipalities | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	814	359
Gross Unrealized Gains	247	110
Gross Unrealized Losses	0	0
Fair Value	1,061	469
Mortgage- and asset-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	20	14
Fair Value	21	15
Mortgage- and asset-backed securities | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	20	14
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Fair Value	21	15
Public utilities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,728	2,727
Fair Value	3,068	2,973
Public utilities | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,728	2,727
Gross Unrealized Gains	340	254
Gross Unrealized Losses	0	8
Fair Value	3,068	2,973
Sovereign and supranational
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,154	1,551
Fair Value	1,332	1,840
Sovereign and supranational | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,154	1,551
Gross Unrealized Gains	178	289
Gross Unrealized Losses	0	0
Fair Value	1,332	1,840
Banks/financial institutions
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	949	1,445
Fair Value	1,039	1,583
Banks/financial institutions | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	949	1,445
Gross Unrealized Gains	100	158
Gross Unrealized Losses	10	20
Fair Value	1,039	1,583
Other corporate
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,537	2,510
Fair Value	2,946	2,804
Other corporate | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,537	2,510
Gross Unrealized Gains	414	332
Gross Unrealized Losses	5	38
Fair Value	$ 2,946	$ 2,804