Fair Value Measurements (Assets Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 44	$ 41
Transfers Into Level 3, Asset	0	0
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	1	0
Net Realized Gains/ (Losses), Assets	2	0
Purchases, Assets	17	0
Sales, Assets	0	0
Settlements, Assets	0	0
Balance-End of Period, Assets	64	41
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	0	25
Transfers Into Level 3, Asset	5	0
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Net Realized Gains/ (Losses), Assets	0	0
Purchases, Assets	8	7
Sales, Assets	0	0
Settlements, Assets	(1)	(11)
Balance-End of Period, Assets	12	21
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		0
Other Long-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	263	225
Transfers Into Level 3, Asset	0	0
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	2	4
Net Realized Gains/ (Losses), Assets	0	0
Purchases, Assets	14	8
Sales, Assets	0	0
Settlements, Assets	(9)	(4)
Balance-End of Period, Assets	270	233
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Available-for-sale Securities [Member] | Foreign Government Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	93	74
Transfers Into Level 3, Asset	7	0
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	9	(1)
Net Realized Gains/ (Losses), Assets	0	(1)
Purchases, Assets	87	14
Sales, Assets	(19)	(3)
Settlements, Assets	(1)	(3)
Balance-End of Period, Assets	176	80
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Available-for-sale Securities [Member] | Corporate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	1,037	681
Transfers Into Level 3, Asset	0	29
Transfers out of Level 3, Assets	(10)	(54)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	(3)	(8)
Net Realized Gains/ (Losses), Assets	0	(1)
Purchases, Assets	139	156
Sales, Assets	(51)	(27)
Settlements, Assets	(39)	(39)
Balance-End of Period, Assets	1,073	737
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Available-for-sale Securities [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	78	45
Transfers Into Level 3, Asset	1	0
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Net Realized Gains/ (Losses), Assets	0	0
Purchases, Assets	4	1
Sales, Assets	0	(1)
Settlements, Assets	0	0
Balance-End of Period, Assets	83	45
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 0	$ 0