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, 2018		Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 82		$ 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	1		7	5	8
Sales, Assets	0		0	0	(1)
Settlements, Assets	(18)		(8)	(19)	(8)
Balance-End of Period, Assets	65		44	65	44
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0		0	0	0
Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	1,181		747	1,037	681
Transfers Into Level 3, Asset	18		111	24	168
Transfers out of Level 3, Assets	(21)		(26)	(31)	(93)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	7		(1)	(5)	(9)
Net Realized Gains/ (Losses), Assets	(6)		0	(4)	(1)
Purchases, Assets	98	[1]	169	454	390
Sales, Assets	(18)		(24)	(114)	(79)
Settlements, Assets	(85)		(57)	(187)	(138)
Balance-End of Period, Assets	1,174		919	1,174	919
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(6)		0	(6)	0
Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	252		85	93	74
Transfers Into Level 3, Asset	5		0	12	0
Transfers out of Level 3, Assets	(2)		(3)	(2)	(3)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	(2)		1	0	3
Net Realized Gains/ (Losses), Assets	(2)		0	(2)	1
Purchases, Assets	98		24	280	57
Sales, Assets	(22)		(14)	(52)	(36)
Settlements, Assets	(4)		(3)	(6)	(6)
Balance-End of Period, Assets	323		90	323	90
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(1)		0	(1)	0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	59		39	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)	(1)		0	0	1
Net Realized Gains/ (Losses), Assets	7		1	6	1
Purchases, Assets	6		17	26	23
Sales, Assets	(18)		(5)	(23)	(14)
Settlements, Assets	0		0	0	0
Balance-End of Period, Assets	53		52	53	52
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	1		0	0	0
Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	12		7	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	0		1	9	15
Sales, Assets	0		0	0	0
Settlements, Assets	(6)		(8)	(8)	(40)
Balance-End of Period, Assets	6		0	6	0
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	264		243	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)	(4)		0	(2)	3
Net Realized Gains/ (Losses), Assets	0		0	1	0
Purchases, Assets	20		15	50	39
Sales, Assets	0		0	0	0
Settlements, Assets	(17)		(6)	(49)	(15)
Balance-End of Period, Assets	263		252	263	252
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 0		$ 0	$ 1	$ 0

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