Description of business and significant accounting policies and practices (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Accounting Policies [Line Items]
Net income	$ 446	$ 672	$ 711	$ 1,338
Basic EPS:
Income allocated to common stock for basic EPS calculation	438	661	699	1,317
Weighted average number of shares outstanding, basic (in shares)	1,140	1,156	1,142	1,161
Basic EPS (in dollars per share)	$ 0.38	$ 0.57	$ 0.61	$ 1.13
Adjustment for Dilutive Shares:
Stock-based compensation plans (in shares)	14	24	17	26
Diluted EPS:
Income allocated to common stock for diluted EPS calculation	438	662	699	1,317
Weighted average number of shares outstanding, diluted (in shares)	1,154	1,180	1,159	1,187
Diluted EPS (in dollars per share)	$ 0.38	$ 0.56	$ 0.60	$ 1.11
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)	53	10	51	8
Basic EPS [Member] | Less income allocated to RSUs [Member]
Accounting Policies [Line Items]
Net income	(8)	(11)	(12)	(21)
Diluted EPS [Member] | Less income allocated to RSUs [Member]
Accounting Policies [Line Items]
Net income	$ (8)	$ (10)	$ (12)	$ (21)