Available-for-sale investments (Schedule) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 41,453	$ 37,911
Gross unrealized gains	87	128
Gross unrealized losses	(222)	(258)
Fair value	41,318	37,781
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	8,313	6,681
Gross unrealized gains	1	1
Gross unrealized losses	(72)	(68)
Fair value	8,242	6,614
Other government-related debt securities - U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	225	302
Gross unrealized gains	0	0
Gross unrealized losses	(2)	(3)
Fair value	223	299
Other government-related debt securities - Foreign and other [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,415	1,784
Gross unrealized gains	18	9
Gross unrealized losses	(11)	(34)
Fair value	2,422	1,759
Corporate debt securities - Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	10,089	8,476
Gross unrealized gains	17	21
Gross unrealized losses	(34)	(37)
Fair value	10,072	8,460
Corporate debt securities - Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	9,688	8,793
Gross unrealized gains	34	59
Gross unrealized losses	(52)	(63)
Fair value	9,670	8,789
Corporate debt securities - Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,393	1,079
Gross unrealized gains	3	5
Gross unrealized losses	(6)	(7)
Fair value	1,390	1,077
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,198	1,968
Gross unrealized gains	0	1
Gross unrealized losses	(30)	(29)
Fair value	2,168	1,940
Other mortgage- and asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,312	1,731
Gross unrealized gains	0	1
Gross unrealized losses	(15)	(13)
Fair value	2,297	1,719
Money market mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,245	2,782
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	3,245	2,782
Other short-term interest-bearing securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,440	4,188
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	1,440	4,188
Total interest-bearing securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	41,318	37,784
Gross unrealized gains	73	97
Gross unrealized losses	(222)	(254)
Fair value	41,169	37,627
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	135	127
Gross unrealized gains	14	31
Gross unrealized losses	0	(4)
Fair value	$ 149	$ 154