Fair value measurements Fair Value Measurements (Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets		$ 0
Transfers into Level 3, assets		0
Balance- End of year, assets			$ 0
Other Long-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	$ 225	212	204
Transfers into Level 3, assets	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	6	(2)	(6)
Net Realized Gains/Losses, Assets	0	1
Purchased, assets	56	33	33
Sales, assets	0
Settlements, assets	(24)	(19)	(19)
Balance- End of year, assets	263	225	212
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	1	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	25
Transfers into Level 3, assets	0	0
Transfers out of Level 3, assets		(50)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0
Net Realized Gains/Losses, Assets	0	0
Purchased, assets	16	75
Sales, assets	0	0
Settlements, assets	(41)	0
Balance- End of year, assets		25
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	41	16	2
Transfers into Level 3, assets	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(1)	2	3
Net Realized Gains/Losses, Assets	2	1	(2)
Purchased, assets	24	27	13
Sales, assets	(22)	(5)
Settlements, assets	0
Balance- End of year, assets	44	41	16
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(1)		(2)
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 year, assets	45	53	15
Transfers into Level 3, assets	50	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	(1)
Net Realized Gains/Losses, Assets	0	0
Purchased, assets	8	1	41
Sales, assets	(1)	(8)	(2)
Settlements, assets	(24)		(1)
Balance- End of year, assets	78	45	53
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	0
Corporate securities | Available-for-sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	681	174	187
Transfers into Level 3, assets	231	53	16
Transfers out of Level 3, assets	(93)	(10)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(12)	15	(1)
Net Realized Gains/Losses, Assets	0	(13)	(4)
Purchased, assets	521	566	52
Sales, assets	(111)	(59)	(28)
Settlements, assets	(180)	(45)	(48)
Balance- End of year, assets	1,037	681	174
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(2)	(11)	(2)
Foreign Government Debt Securities [Member] | Available-for-sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	74	57	22
Transfers into Level 3, assets	0	9	34
Transfers out of Level 3, assets	(3)	(24)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	3	1	(2)
Net Realized Gains/Losses, Assets	0	(6)	(1)
Purchased, assets	84	70	15
Sales, assets	(59)	(17)	(3)
Settlements, assets	(6)	(16)	(8)
Balance- End of year, assets	93	74	57
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	$ (1)	$ (5)	$ (1)