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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic EPS:
Net income	$ 968	$ 798	$ 2,415	$ 2,150
Income allocated to RSUs	(11)	(11)	(30)	(31)
Income allocated to common stock for basic EPS calculation	$ 957	$ 787	$ 2,385	$ 2,119
Weighted average number of shares outstanding, basic (in shares)	1,003	1,023	1,004	1,036
Basic earnings per common share	$ 0.95	$ 0.77	$ 2.37	$ 2.05
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	14	12	13	13
Diluted EPS:
Net income	$ 968	$ 798	$ 2,415	$ 2,150
Income allocated to RSUs	(11)	(11)	(29)	(30)
Income allocated to common stock for diluted EPS	$ 957	$ 787	$ 2,386	$ 2,120
Weighted average number of shares outstanding, diluted (in shares)	1,017	1,035	1,017	1,049
Diluted earnings per common share	$ 0.94	$ 0.76	$ 2.34	$ 2.02