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, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Long-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	$ 212	$ 204	$ 196
Transfers into Level 3, assets	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(2)	(6)	(1)
Net Realized Gains/Losses, Assets	1
Purchased, assets	33	33	20
Sales, assets	0
Settlements, assets	(19)	(19)	(11)
Balance- End of year, assets	225	212	204
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	1	0	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets		0	7
Transfers into Level 3, assets	0
Transfers out of Level 3, assets	(50)		(7)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0		0
Net Realized Gains/Losses, Assets	0		0
Purchased, assets	75		0
Sales, assets	0		0
Settlements, assets	0		0
Balance- End of year, assets	25		0
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	16	2	4
Transfers into Level 3, assets	0
Transfers out of Level 3, assets			(2)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	2	3
Net Realized Gains/Losses, Assets	1	(2)
Purchased, assets	27	13	2
Sales, assets	(5)		(2)
Settlements, assets	0
Balance- End of year, assets	41	16	2
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	(2)	0
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	53	15	8
Transfers into Level 3, assets	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(1)	0
Net Realized Gains/Losses, Assets	0	0
Purchased, assets	1	41	8
Sales, assets	(8)	(2)
Settlements, assets	0	(1)	(1)
Balance- End of year, assets	45	53	15
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	0
Corporate 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	174	187	166
Transfers into Level 3, assets	53	16	37
Transfers out of Level 3, assets	(10)		(23)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	15	(1)	(1)
Net Realized Gains/Losses, Assets	(13)	(4)	(5)
Purchased, assets	566	52	73
Sales, assets	(59)	(28)	(38)
Settlements, assets	(45)	(48)	(22)
Balance- End of year, assets	681	174	187
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(11)	(2)	(5)
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	57	22	44
Transfers into Level 3, assets	9	34	10
Transfers out of Level 3, assets	(24)		(34)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	1	(2)	(1)
Net Realized Gains/Losses, Assets	(6)	(1)	(3)
Purchased, assets	70	15	15
Sales, assets	(17)	(3)	(4)
Settlements, assets	(16)	(8)	(5)
Balance- End of year, assets	74	57	22
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	$ (5)	$ (1)	$ (4)