Fair Value Measurements (Assets Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - Level 3 - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018		Jun. 30, 2019	Jun. 30, 2018
Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 360	$ 176		$ 345	$ 93
Transfers Into Level 3, Asset	0	0		3	7
Transfers out of Level 3, Assets	0	0		(15)	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	(7)		6	2
Net Realized Gains/ (Losses), Assets	0	0		(1)	0
Purchases, Assets	43	95		96	182
Sales, Assets	(14)	(11)		(19)	(30)
Settlements, Assets	(18)	(1)		(44)	(2)
Balance-End of Period, Assets	371	252		371	252
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	1,342	1,073		1,299	1,037
Transfers Into Level 3, Asset	10	6		15	6
Transfers out of Level 3, Assets	0	0		0	(10)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	1	(9)		5	(12)
Net Realized Gains/ (Losses), Assets	(1)	2		0	2
Purchases, Assets	121	217	[1]	249	356	[1]
Sales, Assets	(36)	(45)		(73)	(96)
Settlements, Assets	(78)	(63)		(136)	(102)
Balance-End of Period, Assets	1,359	1,181		1,359	1,181
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(1)	0		(1)	0
Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	78	83		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	0	0		18	4
Sales, Assets	(1)	0		(1)	0
Settlements, Assets	(1)	(1)		(2)	(1)
Balance-End of Period, Assets	76	82		76	82
Fair Value, Assets Measured on Recurring Basis, Change in 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	55	64		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	0		2	1
Net Realized Gains/ (Losses), Assets	(1)	(3)		(3)	(1)
Purchases, Assets	5	3		14	20
Sales, Assets	(4)	(5)		(14)	(5)
Settlements, Assets	0	0		0	0
Balance-End of Period, Assets	56	59		56	59
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(1)	(3)		(2)	(1)
Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	0	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	4	1		4	9
Sales, Assets	0	0		0	0
Settlements, Assets	0	(1)		(1)	(2)
Balance-End of Period, Assets	4	12		4	12
Fair Value, Assets Measured on Recurring Basis, Change in 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	270		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	0		0	2
Net Realized Gains/ (Losses), Assets	0	1		0	1
Purchases, Assets	0	16			30
Sales, Assets	0	0		0	0
Settlements, Assets	0	(23)		0	(32)
Balance-End of Period, Assets	11	264		11	264
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 0	$ 1		$ 0	$ 1

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