Fair Value Measurements (Assets Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Foreign [Member] | Available-for-sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 74	$ 57
Transfers Into Level 3, Asset	0	6
Transfers out of Level 3, Assets	0	(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(1)	6
Net Realized Gains/Losses, Assets	(1)	(5)
Purchases, Assets	14	5
Sales, Assets	(3)	(1)
Settlements, Assets	(3)	(4)
Balance-End of Period, Assets	80	62
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	(4)
Corporate [Member] | Available-for-sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	681	174
Transfers Into Level 3, Asset	29	16
Transfers out of Level 3, Assets	(54)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(8)	2
Net Realized Gains/Losses, Assets	(1)	(6)
Purchases, Assets	156	93
Sales, Assets	(27)	(14)
Settlements, Assets	(39)	(4)
Balance-End of Period, Assets	737	261
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	(7)
Collateralized Mortgage Backed Securities [Member] | Available-for-sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	45	53
Transfers Into Level 3, Asset	0	0
Transfers out of Level 3, Assets	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Net Realized Gains/Losses, Assets	0	0
Purchases, Assets	1	0
Sales, Assets	(1)	(5)
Settlements, Assets	0	0
Balance-End of Period, Assets	45	48
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	41	16
Transfers Into Level 3, Asset	0	0
Transfers out of Level 3, Assets	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Net Realized Gains/Losses, Assets	0	0
Purchases, Assets	0	13
Sales, Assets	0	0
Settlements, Assets	0	0
Balance-End of Period, Assets	41	29
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	25
Transfers Into Level 3, Asset	0
Transfers out of Level 3, Assets	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0
Net Realized Gains/Losses, Assets	0
Purchases, Assets	7
Sales, Assets	0
Settlements, Assets	(11)
Balance-End of Period, Assets	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	225	212
Transfers Into Level 3, Asset	0	0
Transfers out of Level 3, Assets	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	4	0
Net Realized Gains/Losses, Assets	0	0
Purchases, Assets	8	6
Sales, Assets	0	0
Settlements, Assets	(4)	(7)
Balance-End of Period, Assets	233	211
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 0	$ 0