Description of business and significant accounting policies and practices (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Reportable_Segments	Jun. 30, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of Reportable Segments			2
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 660,000,000	$ 446,000,000	$ 1,022,000,000	$ 711,000,000
Basic EPS:
Income allocated to common stock for basic EPS calculation	649,000,000	438,000,000	1,004,000,000	699,000,000
Weighted average number of shares outstanding, basic (in shares)	1,103	1,140	1,105	1,142
Basic EPS (in dollars per share)	$ 0.59	$ 0.38	$ 0.91	$ 0.61
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	14	14	15	17
Diluted EPS:
Income allocated to common stock for diluted EPS calculation	649,000,000	438,000,000	1,004,000,000	699,000,000
Weighted average number of shares outstanding, diluted (in shares)	1,117	1,154	1,120	1,159
Diluted EPS (in dollars per share)	$ 0.58	$ 0.38	$ 0.90	$ 0.60
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)	13	53	23	51
Gain (loss) on settlement of interest rate swap	0
Basic EPS [Member] | Less income allocated to RSUs [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	11,000,000	8,000,000	18,000,000	12,000,000
Diluted EPS [Member] | Less income allocated to RSUs [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 11,000,000	$ 8,000,000	$ 18,000,000	$ 12,000,000