Description of business and significant accounting policies and practices (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Accounting Policies [Line Items]
Net income	$ 265	$ 666
Basic EPS:
Income allocated to common stock for basic EPS calculation	261	656
Weighted average number of shares outstanding, basic (in shares)	1,143	1,167
Basic EPS (in dollars per share)	$ 0.23	$ 0.56
Adjustment for Dilutive Shares:
Stock-based compensation plans (in shares)	22	27
Diluted EPS:
Income allocated to common stock for diluted EPS calculation	261	656
Weighted average number of shares outstanding, diluted (in shares)	1,165	1,194
Diluted EPS (in dollars per share)	$ 0.22	$ 0.55
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)	23	10
Basic EPS [Member] | Less income allocated to RSUs [Member]
Accounting Policies [Line Items]
Net income	(4)	(10)
Diluted EPS [Member] | Less income allocated to RSUs [Member]
Accounting Policies [Line Items]
Net income	$ (4)	$ (10)