Available-for-sale investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	$ 20,709	$ 23,455
Gross unrealized gains	228	312
Gross unrealized losses	(37)	(22)
Total available-for-sale investments	20,900	23,745
U.S. Treasury securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	2,698	4,443
Gross unrealized gains	9	15
Gross unrealized losses	0	0
Total available-for-sale investments	2,707	4,458
Other government-related debt securities - U.S. [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,013	1,018
Gross unrealized gains	6	12
Gross unrealized losses	0	0
Total available-for-sale investments	1,019	1,030
Other government-related debt securities - Foreign and other [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,436	1,549
Gross unrealized gains	35	60
Gross unrealized losses	(5)	(1)
Total available-for-sale investments	1,466	1,608
Corporate debt securities - Financial [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	3,907	3,266
Gross unrealized gains	72	96
Gross unrealized losses	(2)	(1)
Total available-for-sale investments	3,977	3,361
Corporate debt securities - Industrial [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	4,380	4,283
Gross unrealized gains	79	100
Gross unrealized losses	(2)	(3)
Total available-for-sale investments	4,457	4,380
Corporate debt securities - Other [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	470	441
Gross unrealized gains	9	11
Gross unrealized losses	0	0
Total available-for-sale investments	479	452
Residential mortgage-backed securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,816	1,828
Gross unrealized gains	9	9
Gross unrealized losses	(8)	(8)
Total available-for-sale investments	1,817	1,829
Other mortgage- and asset-backed securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,515	1,769
Gross unrealized gains	2	7
Gross unrealized losses	(20)	(9)
Total available-for-sale investments	1,497	1,767
Money market mutual funds [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,742	2,620
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Total available-for-sale investments	1,742	2,620
Other short-term interest-bearing securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,677	2,186
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Total available-for-sale investments	1,677	2,186
Total interest-bearing securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	20,654	23,403
Gross unrealized gains	221	310
Gross unrealized losses	(37)	(22)
Total available-for-sale investments	20,838	23,691
Equity securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	55	52
Gross unrealized gains	7	2
Gross unrealized losses	0	0
Total available-for-sale investments	$ 62	$ 54