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
	Mar. 31, 2016	Mar. 31, 2015
Basic EPS:
Net income	$ 668	$ 656
Income allocated to RSUs	(9)	(9)
Income allocated to common stock for basic EPS calculation	$ 659	$ 647
Weighted average number of shares outstanding, basic (in shares)	1,007	1,046
Basic earnings per common share	$ 0.65	$ 0.62
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	11	15
Diluted EPS:
Net income	$ 668	$ 656
Income allocated to RSUs	(9)	(9)
Income allocated to common stock for diluted EPS	$ 659	$ 647
Weighted average number of shares outstanding, diluted (in shares)	1,018	1,061
Diluted earnings per common share	$ 0.65	$ 0.61