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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic EPS:
Net income	$ 779	$ 696	$ 1,447	$ 1,352
Income allocated to RSUs	(10)	(10)	(19)	(19)
Income allocated to common stock for basic EPS calculation	$ 769	$ 686	$ 1,428	$ 1,333
Weighted average number of shares outstanding, basic (in shares)	1,004	1,038	1,005	1,042
Basic earnings per common share	$ 0.77	$ 0.66	$ 1.42	$ 1.28
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	12	13	12	14
Diluted EPS:
Net income	$ 779	$ 696	$ 1,447	$ 1,352
Income allocated to RSUs	(10)	(10)	(19)	(19)
Income allocated to common stock for diluted EPS	$ 769	$ 686	$ 1,428	$ 1,333
Weighted average number of shares outstanding, diluted (in shares)	1,016	1,051	1,017	1,056
Diluted earnings per common share	$ 0.76	$ 0.65	$ 1.40	$ 1.26