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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic EPS:
Net income	$ 798	$ 826	$ 2,150	$ 1,996
Income allocated to RSUs	(11)	(13)	(31)	(31)
Income allocated to common stock for basic EPS calculation	$ 787	$ 813	$ 2,119	$ 1,965
Weighted average number of shares outstanding, basic (in shares)	1,023	1,060	1,036	1,070
Basic earnings per common share	$ 0.77	$ 0.77	$ 2.05	$ 1.84
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	12	14	13	15
Diluted EPS:
Net income	$ 798	$ 826	$ 2,150	$ 1,996
Income allocated to RSUs	(11)	(13)	(30)	(31)
Income allocated to common stock for diluted EPS	$ 787	$ 813	$ 2,120	$ 1,965
Weighted average number of shares outstanding, diluted (in shares)	1,035	1,074	1,049	1,085
Diluted earnings per common share	$ 0.76	$ 0.76	$ 2.02	$ 1.81