Fair value measurement (Details) - USD ($) $ in Millions	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Assets:
Total available-for-sale investments	$ 29,689		$ 26,732
Fair Value, Measurements, Recurring [Member]
Derivative Assets:
Interest rate swap contracts	50		46
Total assets	29,999		27,170
Derivative Liabilities:
Interest rate swap contracts	24		26
Contingent consideration obligations in connection with business combinations	215		215
Total liabilities	326		257
Fair Value, Measurements, Recurring [Member] | Foreign currency contracts [Member]
Derivative Assets:
Foreign currency contracts	238		360
Derivative Liabilities:
Foreign currency contracts	7		4
Fair Value, Measurements, Recurring [Member] | Cross currency swap contracts [Member]
Derivative Assets:
Foreign currency contracts	22		32
Derivative Liabilities:
Foreign currency contracts	80		12
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities [Member]
Assets:
Total available-for-sale investments	3,610		3,646
Fair Value, Measurements, Recurring [Member] | Other government-related debt securities - U.S. [Member]
Assets:
Total available-for-sale investments	551		528
Fair Value, Measurements, Recurring [Member] | Other government-related debt securities - Foreign and other [Member]
Assets:
Total available-for-sale investments	1,723		1,569
Fair Value, Measurements, Recurring [Member] | Corporate debt securities - Financial [Member]
Assets:
Total available-for-sale investments	7,426		6,041
Fair Value, Measurements, Recurring [Member] | Corporate debt securities - Industrial [Member]
Assets:
Total available-for-sale investments	7,629		6,351
Fair Value, Measurements, Recurring [Member] | Corporate debt securities - Other [Member]
Assets:
Total available-for-sale investments	829		649
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member]
Assets:
Total available-for-sale investments	1,497		1,702
Fair Value, Measurements, Recurring [Member] | Other mortgage- and asset-backed securities [Member]
Assets:
Total available-for-sale investments	2,014		1,796
Fair Value, Measurements, Recurring [Member] | Money market mutual funds [Member]
Assets:
Total available-for-sale investments	2,951		3,004
Fair Value, Measurements, Recurring [Member] | Other short-term interest-bearing securities [Member]
Assets:
Total available-for-sale investments	1,284		1,302
Fair Value, Measurements, Recurring [Member] | Equity securities [Member]
Assets:
Total available-for-sale investments	175		144
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Derivative Assets:
Interest rate swap contracts	0		0
Total assets	6,736		6,794
Derivative Liabilities:
Interest rate swap contracts	0		0
Contingent consideration obligations in connection with business combinations	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		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	3,610		3,646
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	2,951		3,004
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		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	175		144
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Derivative Assets:
Interest rate swap contracts	50		46
Total assets	23,263		20,376
Derivative Liabilities:
Interest rate swap contracts	24		26
Contingent consideration obligations in connection with business combinations	0		0
Total liabilities	111		42
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Foreign currency contracts [Member]
Derivative Assets:
Foreign currency contracts	238		360
Derivative Liabilities:
Foreign currency contracts	7		4
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Cross currency swap contracts [Member]
Derivative Assets:
Foreign currency contracts	22		32
Derivative Liabilities:
Foreign currency contracts	80		12
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	551		528
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	1,723		1,569
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate debt securities - Financial [Member]
Assets:
Total available-for-sale investments	7,426		6,041
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate debt securities - Industrial [Member]
Assets:
Total available-for-sale investments	7,629		6,351
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate debt securities - Other [Member]
Assets:
Total available-for-sale investments	829		649
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Residential mortgage-backed securities [Member]
Assets:
Total available-for-sale investments	1,497		1,702
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	2,014		1,796
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	1,284		1,302
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	0		0
Total assets	0		0
Derivative Liabilities:
Interest rate swap contracts	0		0
Contingent consideration obligations in connection with business combinations	215	$ 215	215	$ 610	$ 596	$ 595
Total liabilities	215		215
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		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		0
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Equity securities [Member]
Assets:
Total available-for-sale investments	$ 0		$ 0