Fair Value Measurements (Assets Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 64	$ 41	$ 44	$ 41
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	1	1	1
Net Realized Gains/ (Losses), Assets	(3)	0	(1)	0
Purchases, Assets	3	6	20	6
Sales, Assets	(5)	(9)	(5)	(9)
Settlements, Assets	0	0	0	0
Balance-End of Period, Assets	59	39	59	39
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(3)	0	(1)	0
Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	12	21	0	25
Transfers Into Level 3, Asset	0	0	5	0
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	7	9	14
Sales, Assets	0	0	0	0
Settlements, Assets	(1)	(21)	(2)	(32)
Balance-End of Period, Assets	12	7	12	7
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	270	233	263	225
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	(1)	2	3
Net Realized Gains/ (Losses), Assets	1	0	1	0
Purchases, Assets	16	16	30	24
Sales, Assets	0	0	0	0
Settlements, Assets	(23)	(5)	(32)	(9)
Balance-End of Period, Assets	264	243	264	243
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	1	0	1	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	176	80	93	74
Transfers Into Level 3, Asset	0	0	7	0
Transfers out of Level 3, Assets	0	0	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	(7)	3	2	2
Net Realized Gains/ (Losses), Assets	0	2	0	1
Purchases, Assets	95	19	182	33
Sales, Assets	(11)	(19)	(30)	(22)
Settlements, Assets	(1)	0	(2)	(3)
Balance-End of Period, Assets	252	85	252	85
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	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	1,073	737	1,037	681
Transfers Into Level 3, Asset	6	28	6	57
Transfers out of Level 3, Assets	0	(13)	(10)	(67)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	(9)	0	(12)	(8)
Net Realized Gains/ (Losses), Assets	2	0	2	(1)
Purchases, Assets	217	65	356	221
Sales, Assets	(45)	(28)	(96)	(55)
Settlements, Assets	(63)	(42)	(102)	(81)
Balance-End of Period, Assets	1,181	747	1,181	747
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	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	83	45	78	45
Transfers Into Level 3, Asset	0	0	1	0
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	0	0	4	1
Sales, Assets	0	0	0	(1)
Settlements, Assets	(1)	0	(1)	0
Balance-End of Period, Assets	82	45	82	45
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 0	$ 0	$ 0	$ 0