Income taxes (Details 3) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets
Noncurrent deferred and refundable income taxes	$ 2,683	$ 2,367
Deferred tax liabilities
Other liabilities	237	335
Deferred income taxes-net	$ 2,446	$ 2,032