Fair value measurement (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Assets:
Total available-for-sale investments	$ 23,745	$ 20,003
Fair Value, Measurements, Recurring [Member]
Derivative Assets:
Interest rate swap contracts		377
Total assets	23,856	20,552
Derivative Liabilities:
Contingent consideration obligations in connection with a business combination	221	190
Total liabilities	286	264
Fair Value, Measurements, Recurring [Member] | Foreign currency contracts [Member]
Derivative Assets:
Foreign currency contracts	46	172
Derivative Liabilities:
Foreign currency contracts	59	48
Fair Value, Measurements, Recurring [Member] | Cross currency swap contracts [Member]
Derivative Assets:
Foreign currency contracts	65
Derivative Liabilities:
Foreign currency contracts	6	26
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities [Member]
Assets:
Total available-for-sale investments	4,458	3,946
Fair Value, Measurements, Recurring [Member] | Other government-related debt securities - U.S. [Member]
Assets:
Total available-for-sale investments	1,030	1,571
Fair Value, Measurements, Recurring [Member] | Other government-related debt securities - Foreign and other [Member]
Assets:
Total available-for-sale investments	1,608	450
Fair Value, Measurements, Recurring [Member] | Corporate debt securities - Financial [Member]
Assets:
Total available-for-sale investments	3,361	2,508
Fair Value, Measurements, Recurring [Member] | Corporate debt securities - Industrial [Member]
Assets:
Total available-for-sale investments	4,380	3,146
Fair Value, Measurements, Recurring [Member] | Corporate debt securities - Other [Member]
Assets:
Total available-for-sale investments	452	289
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member]
Assets:
Total available-for-sale investments	1,829	518
Fair Value, Measurements, Recurring [Member] | Other mortgage- and asset-backed securities [Member]
Assets:
Total available-for-sale investments	1,767	1,267
Fair Value, Measurements, Recurring [Member] | Money market mutual funds [Member]
Assets:
Total available-for-sale investments	2,620	6,266
Fair Value, Measurements, Recurring [Member] | Other short-term interest-bearing securities [Member]
Assets:
Total available-for-sale investments	2,186
Fair Value, Measurements, Recurring [Member] | Equity securities [Member]
Assets:
Total available-for-sale investments	54	42
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Derivative Assets:
Interest rate swap contracts		0
Total assets	7,132	10,254
Derivative Liabilities:
Contingent consideration obligations in connection with a business combination	0	0
Total liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Foreign currency contracts [Member]
Derivative Assets:
Foreign currency contracts	0	0
Derivative Liabilities:
Foreign currency contracts	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Cross currency swap contracts [Member]
Derivative Assets:
Foreign currency contracts	0
Derivative Liabilities:
Foreign currency contracts	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | U.S. Treasury securities [Member]
Assets:
Total available-for-sale investments	4,458	3,946
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Other government-related debt securities - U.S. [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Other government-related debt securities - Foreign and other [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Corporate debt securities - Financial [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Corporate debt securities - Industrial [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Corporate debt securities - Other [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Residential mortgage-backed securities [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Other mortgage- and asset-backed securities [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Money market mutual funds [Member]
Assets:
Total available-for-sale investments	2,620	6,266
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Other short-term interest-bearing securities [Member]
Assets:
Total available-for-sale investments	0
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Equity securities [Member]
Assets:
Total available-for-sale investments	54	42
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Derivative Assets:
Interest rate swap contracts		377
Total assets	16,724	10,298
Derivative Liabilities:
Contingent consideration obligations in connection with a business combination	0	0
Total liabilities	65	74
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Foreign currency contracts [Member]
Derivative Assets:
Foreign currency contracts	46	172
Derivative Liabilities:
Foreign currency contracts	59	48
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Cross currency swap contracts [Member]
Derivative Assets:
Foreign currency contracts	65
Derivative Liabilities:
Foreign currency contracts	6	26
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | U.S. Treasury securities [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Other government-related debt securities - U.S. [Member]
Assets:
Total available-for-sale investments	1,030	1,571
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Other government-related debt securities - Foreign and other [Member]
Assets:
Total available-for-sale investments	1,608	450
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate debt securities - Financial [Member]
Assets:
Total available-for-sale investments	3,361	2,508
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate debt securities - Industrial [Member]
Assets:
Total available-for-sale investments	4,380	3,146
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate debt securities - Other [Member]
Assets:
Total available-for-sale investments	452	289
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Residential mortgage-backed securities [Member]
Assets:
Total available-for-sale investments	1,829	518
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Other mortgage- and asset-backed securities [Member]
Assets:
Total available-for-sale investments	1,767	1,267
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Money market mutual funds [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Other short-term interest-bearing securities [Member]
Assets:
Total available-for-sale investments	2,186
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Equity securities [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Derivative Assets:
Interest rate swap contracts
Total assets		0
Derivative Liabilities:
Contingent consideration obligations in connection with a business combination	221	190
Total liabilities	221	190
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Foreign currency contracts [Member]
Derivative Assets:
Foreign currency contracts	0	0
Derivative Liabilities:
Foreign currency contracts	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Cross currency swap contracts [Member]
Derivative Assets:
Foreign currency contracts	0
Derivative Liabilities:
Foreign currency contracts	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | U.S. Treasury securities [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Other government-related debt securities - U.S. [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Other government-related debt securities - Foreign and other [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Corporate debt securities - Financial [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Corporate debt securities - Industrial [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Corporate debt securities - Other [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Residential mortgage-backed securities [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Other mortgage- and asset-backed securities [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Money market mutual funds [Member]
Assets:
Total available-for-sale investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Other short-term interest-bearing securities [Member]
Assets:
Total available-for-sale investments	0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Equity securities [Member]
Assets:
Total available-for-sale investments	$ 0	$ 0