Basis of presentation and significant accounting policies and practices - Earnings per share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Basic EPS:
Net income	$ 656	$ 487
Income allocated to RSUs	(9)	(7)
Income allocated to common stock for basic EPS calculation	647	480
Weighted average number of shares outstanding, basic (in shares)	1,046	1,081
Basic earnings per common share	$ 0.62	$ 0.44
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	15	15
Diluted EPS:
Net income	656	487
Income allocated to RSUs	(9)	(7)
Income allocated to common stock for diluted EPS	$ 647	$ 480
Weighted average number of shares outstanding, diluted (in shares)	1,061	1,096
Diluted earnings per common share	$ 0.61	$ 0.44
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)	12	10