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, 2013 Reportable_Segments	Mar. 31, 2012
Accounting Policies [Line Items]
Number of Reportable Segments	2
Net income	$ 362	$ 265
Basic EPS:
Income allocated to common stock for basic EPS calculation	355	261
Weighted average number of shares outstanding, basic (in shares)	1,107	1,143
Basic EPS (in dollars per share)	$ 0.32	$ 0.23
Adjustment for Dilutive Shares:
Stock-based compensation plans (in shares)	16	22
Diluted EPS:
Income allocated to common stock for diluted EPS calculation	355	261
Weighted average number of shares outstanding, diluted (in shares)	1,123	1,165
Diluted EPS (in dollars per share)	$ 0.32	$ 0.22
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)	31	23
Basic EPS [Member] | Less income allocated to RSUs [Member]
Accounting Policies [Line Items]
Net income	7	4
Diluted EPS [Member] | Less income allocated to RSUs [Member]
Accounting Policies [Line Items]
Net income	$ 7	$ 4