Fair Value Measurements (Assets and Liabilities, Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - Level 3 - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021		Mar. 31, 2020
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 73		$ 69
Transfers Into Level 3, Asset	0		0
Transfers out of Level 3, Assets			0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0		1
Net Realized Gains/ (Losses), Assets	2		(2)
Purchases, Assets	3		3
Sales, Assets	(3)		(4)
Settlements, Assets	0		0
Other, Assets	0		0
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	2		(2)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0		0
Balance-End of Period, Assets	75		67
Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	5		6
Transfers Into Level 3, Asset	0		0
Transfers out of Level 3, Assets			0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0		0
Net Realized Gains/ (Losses), Assets	0		0
Purchases, Assets	1		1
Sales, Assets	0		0
Settlements, Assets	(5)		(6)
Other, Assets	0		0
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0		0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0		0
Balance-End of Period, Assets	1		1
Other Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	10		10
Transfers Into Level 3, Asset	0		0
Transfers out of Level 3, Assets			0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0		0
Net Realized Gains/ (Losses), Assets	0		0
Purchases, Assets	0		0
Sales, Assets	0		0
Settlements, Assets	0		0
Other, Assets	0		0
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0		0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0		0
Balance-End of Period, Assets	10		10
Available-for-sale Securities [Member] | Non-U.S.
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	546		449
Transfers Into Level 3, Asset	0		0
Transfers out of Level 3, Assets			(3)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	8		(14)
Net Realized Gains/ (Losses), Assets	0		(2)
Purchases, Assets	121		82
Sales, Assets	(7)		(46)
Settlements, Assets	(18)		(1)
Other, Assets	0		0
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0		0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	10		(15)
Balance-End of Period, Assets	650		465
Available-for-sale Securities [Member] | Corporate and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	1,573		1,451
Transfers Into Level 3, Asset	17		91
Transfers out of Level 3, Assets			(1)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	6		45
Net Realized Gains/ (Losses), Assets	(3)		(13)
Purchases, Assets	169		139
Sales, Assets	(25)		(19)
Settlements, Assets	(125)		(101)
Other, Assets	0		0
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(3)		(14)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	11		(44)
Balance-End of Period, Assets	1,612		1,502
Available-for-sale Securities [Member] | Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	60		60
Transfers Into Level 3, Asset	0		0
Transfers out of Level 3, Assets			0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0		0
Net Realized Gains/ (Losses), Assets	0		0
Purchases, Assets	0		0
Sales, Assets	0		0
Settlements, Assets	(9)		0
Other, Assets	0		0
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0		0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0		0
Balance-End of Period, Assets	51		60
Guaranteed Minimum Income Benefit [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance - Beginning of Period, Liabilities	1,089	[1]	897	[2]
Transfers into level 3, liability	0		0
Transfers out of Level 3, Liabilities			0
Change in Net Unrealized Gains (losses) included in OCI, Liabilities	0		0
Net Realized Gains/(Losses), Liabilities	(319)	[1]	685	[2]
Purchases, Liabilities	0		0
Sales, Liabilities	0		0
Settlements, Liabilities	0		0
Other, Liabilities	(10)		9
Balance - End of Period, Liabilities	760	[1]	1,591	[2]
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Liabilities	(319)	[1]	685	[2]
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Liabilities	$ 0		$ 0

[1] Our GLB reinsurance product meets the definition of a derivative instrument for accounting purposes and is accordingly carried at fair value.

[2] Our GLB reinsurance product meets the definition of a derivative instrument for accounting purposes and is accordingly carried at fair value.