Description of business and significant accounting policies and practices (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Accounting Policies [Line Items]
Net income	$ 784	$ 601	$ 1,495	$ 1,938
Basic EPS:
Income allocated to common stock for basic EPS calculation	769	591	1,468	1,908
Weighted average number of shares outstanding, basic (in shares)	1,130	1,144	1,138	1,156
Basic EPS (in dollars per share)	$ 0.68	$ 0.52	$ 1.29	$ 1.65
Adjustment for Dilutive Shares:
Stock-based compensation plans (in shares)	11	13	15	21
Diluted EPS:
Income allocated to common stock for diluted EPS calculation	770	591	1,468	1,909
Weighted average number of shares outstanding, diluted (in shares)	1,141	1,157	1,153	1,177
Diluted EPS (in dollars per share)	$ 0.67	$ 0.51	$ 1.27	$ 1.62
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)	65	55	65	56
Basic EPS [Member] | Less income allocated to RSUs [Member]
Accounting Policies [Line Items]
Net income	(15)	(10)	(27)	(30)
Diluted EPS [Member] | Less income allocated to RSUs [Member]
Accounting Policies [Line Items]
Net income	$ (14)	$ (10)	$ (27)	$ (29)