Available-for-sale investments (Unrealized Losses and Fair Values) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 4,995	$ 11,569
Less than 12 months, Unrealized losses	(49)	(115)
12 months or greater, Fair value	1,104	1,867
12 months or greater, Unrealized losses	(39)	(59)
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	635	1,770
Less than 12 months, Unrealized losses	(1)	(7)
12 months or greater, Fair value	30	171
12 months or greater, Unrealized losses	0	(1)
Other government-related debt securities - U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	86	160
Less than 12 months, Unrealized losses	0	0
12 months or greater, Fair value	35	178
12 months or greater, Unrealized losses	0	(3)
Other government-related debt securities - Foreign and other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	281	514
Less than 12 months, Unrealized losses	(4)	(14)
12 months or greater, Fair value	71	159
12 months or greater, Unrealized losses	(2)	(10)
Corporate debt securities - Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	859	3,150
Less than 12 months, Unrealized losses	(4)	(14)
12 months or greater, Fair value	65	158
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	2,188	3,931
Less than 12 months, Unrealized losses	(27)	(62)
12 months or greater, Fair value	170	222
12 months or greater, Unrealized losses	(2)	(4)
Corporate debt securities - Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	159	354
Less than 12 months, Unrealized losses	(2)	(4)
12 months or greater, Fair value	0	5
12 months or greater, Unrealized losses	0	0
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	320	614
Less than 12 months, Unrealized losses	(2)	(4)
12 months or greater, Fair value	277	413
12 months or greater, Unrealized losses	(4)	(6)
Other mortgage- and asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	462	1,071
Less than 12 months, Unrealized losses	(7)	(8)
12 months or greater, Fair value	456	561
12 months or greater, Unrealized losses	(30)	(33)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	5	5
Less than 12 months, Unrealized losses	(2)	(2)
12 months or greater, Fair value	0	0
12 months or greater, Unrealized losses	$ 0	$ 0