INVESTMENTS - Held-to-Maturity Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021		Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	$ 22,876		$ 24,474
Allowance for Credit Loss	8		10
Net Carrying Amount	22,868	[1]	24,464
Gross Unrealized Gains	5,188		5,935
Gross Unrealized Losses	0		0
Fair Value	28,056		30,399
Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	22,876		24,474
Allowance for Credit Loss	8		10
Net Carrying Amount	22,868		24,464
Gross Unrealized Gains	5,188		5,935
Gross Unrealized Losses	0		0
Fair Value	28,056		30,399
Japan government and agencies | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	21,918		23,448
Allowance for Credit Loss	3		3
Net Carrying Amount	21,915		23,445
Gross Unrealized Gains	4,924		5,625
Gross Unrealized Losses	0		0
Fair Value	26,839		29,070
Municipalities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	352		377
Fair Value	460		499
Municipalities | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	352		377
Allowance for Credit Loss	0		0
Net Carrying Amount	352		377
Gross Unrealized Gains	108		122
Gross Unrealized Losses	0		0
Fair Value	460		499
Public utilities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	44		47
Fair Value	57		61
Public utilities | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	45		48
Allowance for Credit Loss	1		1
Net Carrying Amount	44		47
Gross Unrealized Gains	13		14
Gross Unrealized Losses	0		0
Fair Value	57		61
Sovereign and supranational
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	534		571
Fair Value	670		736
Sovereign and supranational | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	538		577
Allowance for Credit Loss	4		6
Net Carrying Amount	534		571
Gross Unrealized Gains	136		165
Gross Unrealized Losses	0		0
Fair Value	670		736
Other corporate
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	23		24
Fair Value	30		33
Other corporate | Yen-denominated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	23		24
Allowance for Credit Loss	0		0
Net Carrying Amount	23		24
Gross Unrealized Gains	7		9
Gross Unrealized Losses	0		0
Fair Value	$ 30		$ 33

[1] Net of allowance for credit losses