Available-for-sale investments (Unrealized Losses and Fair Values) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 5,515	$ 20,856
Less than 12 months, Unrealized losses	(63)	(250)
12 months or greater, Fair value	1,925	1,887
12 months or greater, Unrealized losses	(65)	(79)
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	454	4,196
Less than 12 months, Unrealized losses	(1)	(24)
12 months or greater, Fair value	0	0
12 months or greater, Unrealized losses	0	0
Other government-related debt securities - U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	82	494
Less than 12 months, Unrealized losses	0	(2)
12 months or greater, Fair value	20	20
12 months or greater, Unrealized losses	0	0
Other government-related debt securities - Foreign and other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	360	1,306
Less than 12 months, Unrealized losses	(6)	(32)
12 months or greater, Fair value	78	56
12 months or greater, Unrealized losses	(3)	(4)
Corporate debt securities - Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,490	5,988
Less than 12 months, Unrealized losses	(8)	(38)
12 months or greater, Fair value	329	228
12 months or greater, Unrealized losses	(3)	(2)
Corporate debt securities - Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,795	5,427
Less than 12 months, Unrealized losses	(35)	(108)
12 months or greater, Fair value	667	679
12 months or greater, Unrealized losses	(20)	(28)
Corporate debt securities - Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	351	807
Less than 12 months, Unrealized losses	(4)	(19)
12 months or greater, Fair value	50	39
12 months or greater, Unrealized losses	(2)	(2)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	195	804
Less than 12 months, Unrealized losses	(1)	(8)
12 months or greater, Fair value	294	304
12 months or greater, Unrealized losses	(5)	(7)
Other mortgage- and asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	788	1,834
Less than 12 months, Unrealized losses	(8)	(19)
12 months or greater, Fair value	487	561
12 months or greater, Unrealized losses	$ (32)	$ (36)