Fair Value Measurements (Assets Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 41	$ 29	$ 41	$ 16
Transfers Into Level 3, Asset	0	0	0	0
Transfers out of Level 3, Assets	0		0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	1	(1)	1	(1)
Net Realized Gains/ (Losses), Assets	0	1	0	1
Purchases, Assets	6	10	6	23
Sales, Assets	(9)	(2)	(9)	(2)
Settlements, Assets	0	0	0	0
Balance-End of Period, Assets	39	37	39	37
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	21	0	25	0
Transfers Into Level 3, Asset	0	0	0	0
Transfers out of Level 3, Assets	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	7	50	14	50
Sales, Assets	0	0	0	0
Settlements, Assets	(21)	0	(32)	0
Balance-End of Period, Assets	7	50	7	50
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		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	233	211	225	212
Transfers Into Level 3, Asset	0	0	0	0
Transfers out of Level 3, Assets	0		0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	(1)	0	3	0
Net Realized Gains/ (Losses), Assets	0	0	0	0
Purchases, Assets	16	8	24	14
Sales, Assets	0	0	0	0
Settlements, Assets	(5)	(3)	(9)	(10)
Balance-End of Period, Assets	243	216	243	216
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	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	80	62	74	57
Transfers Into Level 3, Asset	0	3	0	9
Transfers out of Level 3, Assets	0		0	(2)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	3	3	2	9
Net Realized Gains/ (Losses), Assets	2	(1)	1	(6)
Purchases, Assets	19	27	33	32
Sales, Assets	(19)	(7)	(22)	(8)
Settlements, Assets	0	0	(3)	(4)
Balance-End of Period, Assets	85	87	85	87
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	(1)	0	(5)
Available-for-sale Securities [Member] | Corporate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	737	261	681	174
Transfers Into Level 3, Asset	28	2	57	18
Transfers out of Level 3, Assets	(13)		(67)	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	9	(8)	11
Net Realized Gains/ (Losses), Assets	0	(2)	(1)	(8)
Purchases, Assets	65	31	221	124
Sales, Assets	(28)	(16)	(55)	(30)
Settlements, Assets	(42)	(4)	(81)	(8)
Balance-End of Period, Assets	747	281	747	281
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	(7)
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	45	48	45	53
Transfers Into Level 3, Asset	0	0	0	0
Transfers out of Level 3, Assets	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	0	1	1	1
Sales, Assets	0	0	(1)	(5)
Settlements, Assets	0	0	0	0
Balance-End of Period, Assets	45	49	45	49
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 0	$ 0	$ 0	$ 0