Earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income (Numerator):
Net income for basic and diluted EPS	$ (4,264)	$ 2,021	$ 2,151	$ 2,071	$ 1,935	$ 2,017	$ 1,870	$ 1,900	$ 1,979	$ 7,722	$ 6,939
Shares (Denominator):
Weighted-average shares for basic EPS									731	748	758
Effect of dilutive securities (in shares)									4	6	8
Weighted-average shares for diluted EPS									735	754	766
Basic EPS (in usd per share)	$ (5.89)	$ 2.78	$ 2.93	$ 2.81	$ 2.61	$ 2.70	$ 2.49	$ 2.52	$ 2.71	$ 10.32	$ 9.15
Diluted EPS (in usd per share)	$ (5.89)	$ 2.76	$ 2.91	$ 2.79	$ 2.59	$ 2.68	$ 2.47	$ 2.50	$ 2.69	$ 10.24	$ 9.06