Available-for-sale investments (Unrealized Losses and Fair Values) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 1,864	$ 20,856
Less than 12 months, Unrealized losses	(16)	(250)
12 months or greater, Fair value	1,444	1,887
12 months or greater, Unrealized losses	(36)	(79)
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	174	4,196
Less than 12 months, Unrealized losses	0	(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		494
Less than 12 months, Unrealized losses		(2)
12 months or greater, Fair value		20
12 months or greater, Unrealized losses		0
Other government-related debt securities - Foreign and other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	53	1,306
Less than 12 months, Unrealized losses	(1)	(32)
12 months or greater, Fair value	67	56
12 months or greater, Unrealized losses	(1)	(4)
Corporate debt securities - Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	420	5,988
Less than 12 months, Unrealized losses	(2)	(38)
12 months or greater, Fair value	171	228
12 months or greater, Unrealized losses	(1)	(2)
Corporate debt securities - Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	884	5,427
Less than 12 months, Unrealized losses	(8)	(108)
12 months or greater, Fair value	405	679
12 months or greater, Unrealized losses	(5)	(28)
Corporate debt securities - Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	64	807
Less than 12 months, Unrealized losses	0	(19)
12 months or greater, Fair value	31	39
12 months or greater, Unrealized losses	(1)	(2)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	100	804
Less than 12 months, Unrealized losses	(1)	(8)
12 months or greater, Fair value	289	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	168	1,834
Less than 12 months, Unrealized losses	(4)	(19)
12 months or greater, Fair value	481	561
12 months or greater, Unrealized losses	(23)	$ (36)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	1
Less than 12 months, Unrealized losses	0
12 months or greater, Fair value	0
12 months or greater, Unrealized losses	$ 0