Gross Unrealized Losses and Fair Value of Corporate Securities Based on Industries (Detail) $ in Millions	Dec. 31, 2020 USD ($) Securities	Dec. 31, 2019 USD ($) Securities
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 1,118	$ 1,452
Less than 12 months, Gross unrealized losses	$ (43)	$ (18)
Less than 12 months, Number of securities in a continuous loss position | Securities	204	231
12 months or more, Fair value	$ 247	$ 817
12 months or more, Gross unrealized losses	$ (7)	$ (25)
12 months or more, Number of securities in a continuous loss position | Securities	54	158
Fair value	$ 1,365	$ 2,269
Gross unrealized losses	$ (50)	$ (43)
Number of securities in a continuous loss position | Securities	258	389
Fixed maturity securities
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 1,118	$ 1,452
Less than 12 months, Gross unrealized losses	$ (43)	$ (18)
Less than 12 months, Number of securities in a continuous loss position | Securities	204	231
12 months or more, Fair value	$ 247	$ 817
12 months or more, Gross unrealized losses	$ (7)	$ (25)
12 months or more, Number of securities in a continuous loss position | Securities	54	158
Fair value	$ 1,365	$ 2,269
Gross unrealized losses	$ (50)	$ (43)
Number of securities in a continuous loss position | Securities	258	389
Fixed maturity securities | U.S. corporate
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 345	$ 123
Less than 12 months, Gross unrealized losses	$ (20)	$ (5)
Less than 12 months, Number of securities in a continuous loss position | Securities	59	27
12 months or more, Fair value	$ 33	$ 302
12 months or more, Gross unrealized losses	$ (3)	$ (13)
12 months or more, Number of securities in a continuous loss position | Securities	4	33
Fair value	$ 378	$ 425
Gross unrealized losses	$ (23)	$ (18)
Number of securities in a continuous loss position | Securities	63	60
Fixed maturity securities | U.S. corporate | Utilities
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 49
Less than 12 months, Gross unrealized losses	$ (2)
Less than 12 months, Number of securities in a continuous loss position | Securities	9
12 months or more, Fair value	$ 0
12 months or more, Gross unrealized losses	$ 0
12 months or more, Number of securities in a continuous loss position | Securities	0
Fair value	$ 49
Gross unrealized losses	$ (2)
Number of securities in a continuous loss position | Securities	9
Fixed maturity securities | U.S. corporate | Energy
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 106	$ 54
Less than 12 months, Gross unrealized losses	$ (13)	$ (3)
Less than 12 months, Number of securities in a continuous loss position | Securities	19	10
12 months or more, Fair value	$ 33	$ 80
12 months or more, Gross unrealized losses	$ (3)	$ (5)
12 months or more, Number of securities in a continuous loss position | Securities	4	10
Fair value	$ 139	$ 134
Gross unrealized losses	$ (16)	$ (8)
Number of securities in a continuous loss position | Securities	23	20
Fixed maturity securities | U.S. corporate | Finance and insurance
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 128	$ 0
Less than 12 months, Gross unrealized losses	$ (2)	$ 0
Less than 12 months, Number of securities in a continuous loss position | Securities	15	0
12 months or more, Fair value	$ 0	$ 34
12 months or more, Gross unrealized losses	$ 0	$ (1)
12 months or more, Number of securities in a continuous loss position | Securities	0	4
Fair value	$ 128	$ 34
Gross unrealized losses	$ (2)	$ (1)
Number of securities in a continuous loss position | Securities	15	4
Fixed maturity securities | U.S. corporate | Consumer-non-cyclical
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 16	$ 34
Less than 12 months, Gross unrealized losses	$ (1)	$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities	5	9
12 months or more, Fair value	$ 0	$ 93
12 months or more, Gross unrealized losses	$ 0	$ (3)
12 months or more, Number of securities in a continuous loss position | Securities	0	9
Fair value	$ 16	$ 127
Gross unrealized losses	$ (1)	$ (4)
Number of securities in a continuous loss position | Securities	5	18
Fixed maturity securities | U.S. corporate | Technology and communications
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value		$ 0
Less than 12 months, Gross unrealized losses		$ 0
Less than 12 months, Number of securities in a continuous loss position | Securities		0
12 months or more, Fair value		$ 18
12 months or more, Gross unrealized losses		$ (1)
12 months or more, Number of securities in a continuous loss position | Securities		2
Fair value		$ 18
Gross unrealized losses		$ (1)
Number of securities in a continuous loss position | Securities		2
Fixed maturity securities | U.S. corporate | Capital goods
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value		$ 35
Less than 12 months, Gross unrealized losses		$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities		8
12 months or more, Fair value		$ 0
12 months or more, Gross unrealized losses		$ 0
12 months or more, Number of securities in a continuous loss position | Securities		0
Fair value		$ 35
Gross unrealized losses		$ (1)
Number of securities in a continuous loss position | Securities		8
Fixed maturity securities | U.S. corporate | Consumer-cyclical
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value		$ 0
Less than 12 months, Gross unrealized losses		$ 0
Less than 12 months, Number of securities in a continuous loss position | Securities		0
12 months or more, Fair value		$ 54
12 months or more, Gross unrealized losses		$ (2)
12 months or more, Number of securities in a continuous loss position | Securities		6
Fair value		$ 54
Gross unrealized losses		$ (2)
Number of securities in a continuous loss position | Securities		6
Fixed maturity securities | U.S. corporate | Transportation
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 46	$ 0
Less than 12 months, Gross unrealized losses	$ (2)	$ 0
Less than 12 months, Number of securities in a continuous loss position | Securities	11	0
12 months or more, Fair value	$ 0	$ 23
12 months or more, Gross unrealized losses	$ 0	$ (1)
12 months or more, Number of securities in a continuous loss position | Securities	0	2
Fair value	$ 46	$ 23
Gross unrealized losses	$ (2)	$ (1)
Number of securities in a continuous loss position | Securities	11	2
Fixed maturity securities | Non-U.S. corporate
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 145	$ 79
Less than 12 months, Gross unrealized losses	$ (4)	$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities	32	12
12 months or more, Fair value	$ 6	$ 62
12 months or more, Gross unrealized losses	$ (1)	$ (4)
12 months or more, Number of securities in a continuous loss position | Securities	1	7
Fair value	$ 151	$ 141
Gross unrealized losses	$ (5)	$ (5)
Number of securities in a continuous loss position | Securities	33	19
Fixed maturity securities | Non-U.S. corporate | Energy
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 66
Less than 12 months, Gross unrealized losses	$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities	10
12 months or more, Fair value	$ 0
12 months or more, Gross unrealized losses	$ 0
12 months or more, Number of securities in a continuous loss position | Securities	0
Fair value	$ 66
Gross unrealized losses	$ (1)
Number of securities in a continuous loss position | Securities	10
Fixed maturity securities | Non-U.S. corporate | Consumer-non-cyclical
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 0	$ 0
Less than 12 months, Gross unrealized losses	$ 0	$ 0
Less than 12 months, Number of securities in a continuous loss position | Securities	0	0
12 months or more, Fair value	$ 6	$ 31
12 months or more, Gross unrealized losses	$ (1)	$ (2)
12 months or more, Number of securities in a continuous loss position | Securities	1	3
Fair value	$ 6	$ 31
Gross unrealized losses	$ (1)	$ (2)
Number of securities in a continuous loss position | Securities	1	3
Fixed maturity securities | Non-U.S. corporate | Capital goods
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 31
Less than 12 months, Gross unrealized losses	$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities	8
12 months or more, Fair value	$ 0
12 months or more, Gross unrealized losses	$ 0
12 months or more, Number of securities in a continuous loss position | Securities	0
Fair value	$ 31
Gross unrealized losses	$ (1)
Number of securities in a continuous loss position | Securities	8
Fixed maturity securities | Non-U.S. corporate | Consumer-cyclical
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 15
Less than 12 months, Gross unrealized losses	$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities	6
12 months or more, Fair value	$ 0
12 months or more, Gross unrealized losses	$ 0
12 months or more, Number of securities in a continuous loss position | Securities	0
Fair value	$ 15
Gross unrealized losses	$ (1)
Number of securities in a continuous loss position | Securities	6
Fixed maturity securities | Non-U.S. corporate | Transportation
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 33	$ 0
Less than 12 months, Gross unrealized losses	$ (1)	$ 0
Less than 12 months, Number of securities in a continuous loss position | Securities	8	0
12 months or more, Fair value	$ 0	$ 25
12 months or more, Gross unrealized losses	$ 0	$ (1)
12 months or more, Number of securities in a continuous loss position | Securities	0	3
Fair value	$ 33	$ 25
Gross unrealized losses	$ (1)	$ (1)
Number of securities in a continuous loss position | Securities	8	3
Fixed maturity securities | Non-U.S. corporate | Other
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value		$ 79
Less than 12 months, Gross unrealized losses		$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities		12
12 months or more, Fair value		$ 6
12 months or more, Gross unrealized losses		$ (1)
12 months or more, Number of securities in a continuous loss position | Securities		1
Fair value		$ 85
Gross unrealized losses		$ (2)
Number of securities in a continuous loss position | Securities		13
Fixed maturity securities | Corporate Debt Securities
Available for Sale Securities Continuous Unrealized Loss Position [Line Items]
Less than 12 months, Fair value	$ 490	$ 202
Less than 12 months, Gross unrealized losses	$ (24)	$ (6)
Less than 12 months, Number of securities in a continuous loss position | Securities	91	39
12 months or more, Fair value	$ 39	$ 364
12 months or more, Gross unrealized losses	$ (4)	$ (17)
12 months or more, Number of securities in a continuous loss position | Securities	5	40
Fair value	$ 529	$ 566
Gross unrealized losses	$ (28)	$ (23)
Number of securities in a continuous loss position | Securities	96	79