Basis of presentation and significant accounting policies and practices - Earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic EPS:
Net income	$ 696	$ 683	$ 1,352	$ 1,170
Income allocated to RSUs	(10)	(10)	(19)	(18)
Income allocated to common stock for basic EPS calculation	$ 686	$ 673	$ 1,333	$ 1,152
Weighted average number of shares outstanding, basic (in shares)	1,038	1,071	1,042	1,076
Basic earnings per common share	$ 0.66	$ 0.63	$ 1.28	$ 1.07
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	13	15	14	15
Diluted EPS:
Net income	$ 696	$ 683	$ 1,352	$ 1,170
Income allocated to RSUs	(10)	(10)	(19)	(18)
Income allocated to common stock for diluted EPS	$ 686	$ 673	$ 1,333	$ 1,152
Weighted average number of shares outstanding, diluted (in shares)	1,051	1,086	1,056	1,091
Diluted earnings per common share	$ 0.65	$ 0.62	$ 1.26	$ 1.06