Available-for-sale investments (Unrealized Losses and Fair Values) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 4,182	$ 20,856
Less than 12 months, Unrealized losses	(18)	(250)
12 months or greater, Fair value	578	1,887
12 months or greater, Unrealized losses	(7)	(79)
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	957	4,196
Less than 12 months, Unrealized losses	(3)	(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	77	494
Less than 12 months, Unrealized losses	0	(2)
12 months or greater, Fair value	0	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	192	1,306
Less than 12 months, Unrealized losses	(2)	(32)
12 months or greater, Fair value	18	56
12 months or greater, Unrealized losses	0	(4)
Corporate debt securities - Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	638	5,988
Less than 12 months, Unrealized losses	(2)	(38)
12 months or greater, Fair value	47	228
12 months or greater, Unrealized losses	0	(2)
Corporate debt securities - Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1,275	5,427
Less than 12 months, Unrealized losses	(6)	(108)
12 months or greater, Fair value	204	679
12 months or greater, Unrealized losses	(2)	(28)
Corporate debt securities - Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	110	807
Less than 12 months, Unrealized losses	(1)	(19)
12 months or greater, Fair value	25	39
12 months or greater, Unrealized losses	0	(2)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	667	804
Less than 12 months, Unrealized losses	(2)	(8)
12 months or greater, Fair value	182	304
12 months or greater, Unrealized losses	(3)	(7)
Other mortgage- and asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	266	1,834
Less than 12 months, Unrealized losses	(2)	(19)
12 months or greater, Fair value	102	561
12 months or greater, Unrealized losses	$ (2)	$ (36)