Stock-based compensation (Components of Stock-based Compensation Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of stock-based compensation expense [Abstract]
Total stock-based compensation expense, pretax	$ 329	$ 311	$ 322
Tax benefit from stock-based compensation expense	(118)	(112)	(120)
Total stock-based compensation expense, net of tax	211	199	202
RSUs [Member]
Components of stock-based compensation expense [Abstract]
Total stock-based compensation expense, pretax	174	177	190
Performance units [Member]
Components of stock-based compensation expense [Abstract]
Total stock-based compensation expense, pretax	133	123	132
Stock options [Member]
Components of stock-based compensation expense [Abstract]
Total stock-based compensation expense, pretax	$ 22	$ 11	$ 0