Fair value measurements (Details Textual) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Length of time hedged in foreign currency contracts (or less)	3 years
Long-term debt, fair value	$ 37,600	$ 38,600
Carrying value of debt	$ 35,541	$ 35,342
Other Government-related and Corporate Debt Securities [Member]
Business Acquisition [Line Items]
Investment maturity period	5 years