Available-for-sale investments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	$ 24,609	$ 19,814
Gross unrealized gains	344	224
Gross unrealized losses	(18)	(35)
Estimated fair value	24,935	20,003
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	3,936	3,878
Gross unrealized gains	24	68
Gross unrealized losses	0	0
Estimated fair value	3,960	3,946
Other government-related debt securities - Obligations of U.S. government agencies and FDIC-guaranteed bank debt [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,180	1,548
Gross unrealized gains	18	23
Gross unrealized losses	0	0
Estimated fair value	1,198	1,571
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	1,656	441
Gross unrealized gains	56	9
Gross unrealized losses	0	0
Estimated fair value	1,712	450
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	3,158	2,493
Gross unrealized gains	99	30
Gross unrealized losses	(1)	(15)
Estimated fair value	3,256	2,508
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	4,208	3,077
Gross unrealized gains	114	79
Gross unrealized losses	(4)	(10)
Estimated fair value	4,318	3,146
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	420	280
Gross unrealized gains	12	9
Gross unrealized losses	0	0
Estimated fair value	432	289
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	1,834	518
Gross unrealized gains	9	3
Gross unrealized losses	(7)	(3)
Estimated fair value	1,836	518
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	2,062	1,271
Gross unrealized gains	10	3
Gross unrealized losses	(6)	(7)
Estimated fair value	2,066	1,267
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	4,000	6,266
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	4,000	6,266
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	2,105
Gross unrealized gains	0
Gross unrealized losses	0
Estimated fair value	2,105
Total debt security investments [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	24,559	19,772
Gross unrealized gains	342	224
Gross unrealized losses	(18)	(35)
Estimated fair value	24,883	19,961
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	50	42
Gross unrealized gains	2	0
Gross unrealized losses	0	0
Estimated fair value	$ 52	$ 42