Summary of significant accounting policies (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) segment	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Accounting Policies [Abstract]
Number of operating segments | segment		1
Accumulated depreciation and amortization on property, plant and equipment	$ 7,500	$ 7,500		$ 7,300
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Excess tax benefits from share-based payments	$ 37	114
Change in net cash provided by operating activities		4,592	$ 4,766
Change in net cash used in financing activities		$ (1,059)	(1,391)
ASU 2016-09, Excess Tax Benefit [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in net cash provided by operating activities			233
Change in net cash used in financing activities			(233)
ASU 2016-09, Withholding Taxes on Shares Withheld [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in net cash provided by operating activities			390
Change in net cash used in financing activities			$ (390)