Fair Value Measurements (Assets Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - Level 3 - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018		Sep. 30, 2019	Sep. 30, 2018
Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 371	$ 252		$ 345	$ 93
Transfers Into Level 3, Asset	0	5		3	12
Transfers out of Level 3, Assets	0	(2)		(15)	(2)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	(8)	(2)		(2)	0
Net Realized Gains/ (Losses), Assets	0	(2)		(1)	(2)
Purchases, Assets	68	98		164	280
Sales, Assets	(35)	(22)		(54)	(52)
Settlements, Assets	(3)	(4)		(47)	(6)
Balance-End of Period, Assets	393	323		393	323
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0	(1)		0	(1)
Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	1,359	1,181		1,299	1,037
Transfers Into Level 3, Asset	1	18		16	24
Transfers out of Level 3, Assets	0	(21)		0	(31)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	(4)	7		1	(5)
Net Realized Gains/ (Losses), Assets	0	(6)		0	(4)
Purchases, Assets	176	98	[1]	425	454	[1]
Sales, Assets	(18)	(18)		(91)	(114)
Settlements, Assets	(64)	(85)		(200)	(187)
Balance-End of Period, Assets	1,450	1,174		1,450	1,174
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0	(6)		(1)	(6)
Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	76	82		61	78
Transfers Into Level 3, Asset	0	0		0	1
Transfers out of Level 3, Assets	0	0		0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0		0	0
Net Realized Gains/ (Losses), Assets	0	0		0	0
Purchases, Assets	1	1		19	5
Sales, Assets	0	0		(1)	0
Settlements, Assets	0	(18)		(2)	(19)
Balance-End of Period, Assets	77	65		77	65
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0	0		0	0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	56	59		57	44
Transfers Into Level 3, Asset	0	0		0	0
Transfers out of Level 3, Assets	0	0		0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	(1)	(1)		1	0
Net Realized Gains/ (Losses), Assets	(1)	7		(4)	6
Purchases, Assets	5	6		19	26
Sales, Assets	(3)	(18)		(17)	(23)
Settlements, Assets	0	0		0	0
Balance-End of Period, Assets	56	53		56	53
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(1)	1		(3)	0
Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	4	12		1	0
Transfers Into Level 3, Asset	0	0		0	5
Transfers out of Level 3, Assets	0	0		0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0		0	0
Net Realized Gains/ (Losses), Assets	0	0		0	0
Purchases, Assets	2	0		6	9
Sales, Assets	0	0		0	0
Settlements, Assets	0	(6)		(1)	(8)
Balance-End of Period, Assets	6	6		6	6
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0	0		0	0
Other Long-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	11	264		11	263
Transfers Into Level 3, Asset	0	0		0	0
Transfers out of Level 3, Assets	0	0		0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	(4)		0	(2)
Net Realized Gains/ (Losses), Assets	0	0		0	1
Purchases, Assets	0	20		0	50
Sales, Assets	0	0		0	0
Settlements, Assets	(1)	(17)		(1)	(49)
Balance-End of Period, Assets	10	263		10	263
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ 0	$ 0		$ 0	$ 1

[1] Purchases in Level 3 primarily consist of privately-placed fixed income securities.