Operations and summary of significant accounting policies (Details 4) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid expenses and other current assets	$ 1,772	$ 1,682
Noncurrent deferred and refundable income taxes	1,693	2,790
Other current liabilities	1,975	1,635
Other liabilities	$ 4,053	$ 3,184