Earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income (Numerator):
Net income for basic and diluted EPS	$ 1,863	$ 1,244	$ 5,139	$ 3,864
Shares (Denominator):
Weighted-average shares for basic EPS	757	761	760	758
Effect of dilutive securities	7	10	7	11
Weighted-average shares for diluted EPS	764	771	767	769
Basic EPS (in usd per share)	$ 2.46	$ 1.63	$ 6.76	$ 5.10
Diluted EPS (in usd per share)	$ 2.44	$ 1.61	$ 6.70	$ 5.02