Available-for-sale investments (Unrealized Losses and Fair Values) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 25,250	$ 18,281
Less than 12 months, Unrealized losses	(191)	(250)
12 months or greater, Fair value	2,061	493
12 months or greater, Unrealized losses	(31)	(8)
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	7,728	5,774
Less than 12 months, Unrealized losses	(70)	(68)
12 months or greater, Fair value	195	0
12 months or greater, Unrealized losses	(2)	0
Other government-related debt securities - U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	188	201
Less than 12 months, Unrealized losses	(1)	(3)
12 months or greater, Fair value	34	0
12 months or greater, Unrealized losses	(1)	0
Other government-related debt securities - Foreign and other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,163	1,192
Less than 12 months, Unrealized losses	(9)	(34)
12 months or greater, Fair value	115	17
12 months or greater, Unrealized losses	(2)	0
Corporate debt securities - Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	5,928	3,975
Less than 12 months, Unrealized losses	(28)	(37)
12 months or greater, Fair value	462	44
12 months or greater, Unrealized losses	(6)	0
Corporate debt securities - Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	5,760	3,913
Less than 12 months, Unrealized losses	(43)	(61)
12 months or greater, Fair value	612	149
12 months or greater, Unrealized losses	(9)	(2)
Corporate debt securities - Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	868	486
Less than 12 months, Unrealized losses	(4)	(7)
12 months or greater, Fair value	117	7
12 months or greater, Unrealized losses	(2)	0
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,838	1,631
Less than 12 months, Unrealized losses	(24)	(26)
12 months or greater, Fair value	276	158
12 months or greater, Unrealized losses	(6)	(3)
Other mortgage- and asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,777	1,087
Less than 12 months, Unrealized losses	(12)	(10)
12 months or greater, Fair value	250	118
12 months or greater, Unrealized losses	$ (3)	(3)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value		22
Less than 12 months, Unrealized losses		(4)
12 months or greater, Fair value		0
12 months or greater, Unrealized losses		$ 0