INVESTMENTS - Fair Value and Gross Unrealized Losses for Securities That Have Been in Continuous Unrealized Loss Position (Detail) - Fixed maturity securities - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2020
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		$ 11,432
Total Unrealized Losses		638
Less than 12 months Fair Value		7,684
Less than 12 months Unrealized Losses		323
12 months or longer Fair Value		3,748
12 months or longer Unrealized Losses		315
Total Fair Value	$ 7,190
Total Unrealized Losses	385
Less than 12 months Fair Value	3,423
Less than 12 months Unrealized Losses	178
12 months or longer Fair Value	3,767
12 months or longer Unrealized Losses	207
Yen-denominated | Japan government and agencies
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		1,997
Total Unrealized Losses		42
Less than 12 months Fair Value		1,997
Less than 12 months Unrealized Losses		42
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Yen-denominated | Municipalities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		74
Total Unrealized Losses		3
Less than 12 months Fair Value		60
Less than 12 months Unrealized Losses		2
12 months or longer Fair Value		14
12 months or longer Unrealized Losses		1
Total Fair Value	80
Total Unrealized Losses	3
Less than 12 months Fair Value	80
Less than 12 months Unrealized Losses	3
12 months or longer Fair Value	0
12 months or longer Unrealized Losses	0
Yen-denominated | Mortgage- and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		36
Total Unrealized Losses		1
Less than 12 months Fair Value		36
Less than 12 months Unrealized Losses		1
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Yen-denominated | Public utilities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		214
Total Unrealized Losses		1
Less than 12 months Fair Value		214
Less than 12 months Unrealized Losses		1
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Yen-denominated | Sovereign and supranational
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		35
Total Unrealized Losses		4
Less than 12 months Fair Value		35
Less than 12 months Unrealized Losses		4
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Yen-denominated | Banks/financial institutions
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		2,859
Total Unrealized Losses		180
Less than 12 months Fair Value		1,853
Less than 12 months Unrealized Losses		102
12 months or longer Fair Value		1,006
12 months or longer Unrealized Losses		78
Total Fair Value	1,828
Total Unrealized Losses	89
Less than 12 months Fair Value	1,828
Less than 12 months Unrealized Losses	89
12 months or longer Fair Value	0
12 months or longer Unrealized Losses	0
Yen-denominated | Other corporate
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		1,095
Total Unrealized Losses		86
Less than 12 months Fair Value		911
Less than 12 months Unrealized Losses		81
12 months or longer Fair Value		184
12 months or longer Unrealized Losses		5
Total Fair Value	636
Total Unrealized Losses	31
Less than 12 months Fair Value	636
Less than 12 months Unrealized Losses	31
12 months or longer Fair Value	0
12 months or longer Unrealized Losses	0
Dollar-denominated | Mortgage- and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		67
Total Unrealized Losses		1
Less than 12 months Fair Value		67
Less than 12 months Unrealized Losses		1
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Dollar-denominated | Public utilities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		342
Total Unrealized Losses		12
Less than 12 months Fair Value		210
Less than 12 months Unrealized Losses		6
12 months or longer Fair Value		132
12 months or longer Unrealized Losses		6
Total Fair Value	306
Total Unrealized Losses	10
Less than 12 months Fair Value	69
Less than 12 months Unrealized Losses	2
12 months or longer Fair Value	237
12 months or longer Unrealized Losses	8
Dollar-denominated | Sovereign and supranational
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		36
Total Unrealized Losses		6
Less than 12 months Fair Value		36
Less than 12 months Unrealized Losses		6
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Dollar-denominated | Banks/financial institutions
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		103
Total Unrealized Losses		6
Less than 12 months Fair Value		65
Less than 12 months Unrealized Losses		1
12 months or longer Fair Value		38
12 months or longer Unrealized Losses		5
Total Fair Value	79
Total Unrealized Losses	4
Less than 12 months Fair Value	18
Less than 12 months Unrealized Losses	0
12 months or longer Fair Value	61
12 months or longer Unrealized Losses	4
Dollar-denominated | Other corporate
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		4,574
Total Unrealized Losses		296
Less than 12 months Fair Value		2,200
Less than 12 months Unrealized Losses		76
12 months or longer Fair Value		2,374
12 months or longer Unrealized Losses		$ 220
Total Fair Value	4,261
Total Unrealized Losses	248
Less than 12 months Fair Value	792
Less than 12 months Unrealized Losses	53
12 months or longer Fair Value	3,469
12 months or longer Unrealized Losses	$ 195