Income taxes (Details 3) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Noncurrent deferred and refundable income taxes	$ 1,569	$ 2,683
Deferred tax liabilities
Other liabilities	281	237
Deferred income taxes-net	$ 1,288	$ 2,446