INVESTMENTS - Fair Value and Gross Unrealized Losses for Securities That Have Been in Continuous Unrealized Loss Position (Detail) - Fixed maturity securities - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	$ 11,251	$ 10,781
Total Unrealized Losses	409	481
Less than 12 months Fair Value	6,777	6,902
Less than 12 months Unrealized Losses	147	184
12 months or longer Fair Value	4,474	3,879
12 months or longer Unrealized Losses	262	297
Yen-denominated | Japan government and agencies
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	3,207	2,604
Total Unrealized Losses	80	52
Less than 12 months Fair Value	2,529	2,604
Less than 12 months Unrealized Losses	32	52
12 months or longer Fair Value	678	0
12 months or longer Unrealized Losses	48	0
Yen-denominated | Municipalities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	193	183
Total Unrealized Losses	6	5
Less than 12 months Fair Value	140	169
Less than 12 months Unrealized Losses	3	4
12 months or longer Fair Value	53	14
12 months or longer Unrealized Losses	3	1
Yen-denominated | Mortgage- and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	34	37
Total Unrealized Losses	1	1
Less than 12 months Fair Value	26	37
Less than 12 months Unrealized Losses	1	1
12 months or longer Fair Value	8	0
12 months or longer Unrealized Losses	0	0
Yen-denominated | Public utilities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	314	135
Total Unrealized Losses	5	1
Less than 12 months Fair Value	279	135
Less than 12 months Unrealized Losses	5	1
12 months or longer Fair Value	35	0
12 months or longer Unrealized Losses	0	0
Yen-denominated | Banks/financial institutions
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	2,098	1,809
Total Unrealized Losses	103	102
Less than 12 months Fair Value	590	765
Less than 12 months Unrealized Losses	7	36
12 months or longer Fair Value	1,508	1,044
12 months or longer Unrealized Losses	96	66
Yen-denominated | Other corporate
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	811	613
Total Unrealized Losses	35	37
Less than 12 months Fair Value	351	290
Less than 12 months Unrealized Losses	5	13
12 months or longer Fair Value	460	323
12 months or longer Unrealized Losses	30	24
Dollar-denominated | Municipalities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	147	94
Total Unrealized Losses	4	2
Less than 12 months Fair Value	147	94
Less than 12 months Unrealized Losses	4	2
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Dollar-denominated | Mortgage- and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	55	360
Total Unrealized Losses	1	5
Less than 12 months Fair Value	55	360
Less than 12 months Unrealized Losses	1	5
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Dollar-denominated | Public utilities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	323	326
Total Unrealized Losses	9	15
Less than 12 months Fair Value	235	208
Less than 12 months Unrealized Losses	6	7
12 months or longer Fair Value	88	118
12 months or longer Unrealized Losses	3	8
Dollar-denominated | Sovereign and supranational
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	39	39
Total Unrealized Losses	3	3
Less than 12 months Fair Value	0	39
Less than 12 months Unrealized Losses	0	3
12 months or longer Fair Value	39	0
12 months or longer Unrealized Losses	3	0
Dollar-denominated | Banks/financial institutions
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	204	82
Total Unrealized Losses	6	7
Less than 12 months Fair Value	145	44
Less than 12 months Unrealized Losses	2	1
12 months or longer Fair Value	59	38
12 months or longer Unrealized Losses	4	6
Dollar-denominated | Other corporate
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	3,826	4,499
Total Unrealized Losses	156	251
Less than 12 months Fair Value	2,280	2,157
Less than 12 months Unrealized Losses	81	59
12 months or longer Fair Value	1,546	2,342
12 months or longer Unrealized Losses	$ 75	$ 192